Squashed commit of the following:

commit 2d7d545c4c4bfce7fdcbcbe9baaeb437915742f0
Merge: 625a1249 b178914b
Author: Yukihiro "Matz" Matsumoto <matz@ruby.or.jp>
Date:   Fri Jun 5 14:35:13 2020 +0900

    Merge branch 'fix-mrb_open-with-nomem' of https://github.com/dearblue/mruby into dearblue-fix-mrb_open-with-nomem

commit b178914b11
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sat Jan 19 22:22:44 2019 +0900

    Fix invalid pointer free inside other heap's block

     1. `e = mrb_obj_alloc(...)`
     2. `e->stack = mrb->c->stack` (`mrb->c->stack` is anywhere in the range `stbase...stend`)
     3. And raised exception by `mrb_malloc()`!
     4. `mrb_free(e->stack)` by GC part (wrong free)

commit 52e3d5d858
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sat Jan 19 21:55:36 2019 +0900

    Fix memory leak for temporary symbols when out of memory

commit 4c5499b88e
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sun Jan 20 11:42:07 2019 +0900

    Fix uninitialized pointer dereference for debug section

commit 8e993167de
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sun Jan 20 11:41:09 2019 +0900

    Fix memory leak for temporary filenames when out of memory

commit 8b422577e6
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sun Jan 20 10:57:51 2019 +0900

    Fix memory leak for irep when out of memory

commit 6b35ebf49a
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sun Jan 20 10:55:50 2019 +0900

    Fix uninitialized pointer dereference when do not finished initializing irep

commit 2531f2631e
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sun Jan 20 10:48:15 2019 +0900

    Fix NULL pointer dereference when do not finished initializing irep

commit e2d6896eba
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sat Jan 19 12:54:19 2019 +0900

    Fix memory leak for irep when out of memory by `mrb_proc_new()`

commit b6214ff8a0
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sat Jan 19 12:53:07 2019 +0900

    Fix memory leak for `khash_t` in `kh_init_size()` when out of memory by `kh_alloc()`

commit 19162dd6c1
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sun Jan 20 02:15:07 2019 +0900

    Fix memory leak for symbol string when out of memory in `kh_put()`

commit 15e67297ff
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sun Jan 20 02:12:24 2019 +0900

    Fix keep wrong symbol index when out of memory

commit 3f8e2b3752
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sun Jan 20 02:08:13 2019 +0900

    Fix keep wrong symbol capacity when out of memory

commit a3cfe755ab
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sat Jan 19 10:11:37 2019 +0900

    Fix NULL pointer dereference `mrb->c` by `mark_context()`

commit d9c7b6be6e
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sun Jan 20 15:25:09 2019 +0900

    Fix protect exception for print error message

commit 100642750e
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sun Jan 20 11:59:02 2019 +0900

    Protect exception for mruby core initialization

commit 7a0418304e
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Fri Jan 18 20:38:27 2019 +0900

    Fix memory leak for string object when out of memory

    The `mrb_str_pool()` function has a path to call `malloc()` twice.

    If occurs `NoMemoryError` exception in second `malloc()`,
    first `malloc()` pointer is not freed.

commit fef1c152ce
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sat Jan 19 13:05:09 2019 +0900

    Fix stack overflow when out of memory

    As a result of this change, no backtrace information is set
    for NoMemoryError (`mrb->nomem_err`).

    Detailes:

    When generating a backtrace, called `mrb_intern_lit()`,
    `mrb_str_new_cstr()` and `mrb_obj_iv_set()` function with
    `exc_debug_info()` function in `src/error.c`.

    If a `NoMemoryError` exception occurs at this time,
    the `exc_debug_info()` function will be called again,
    and in the same way `NoMemoryError` exception raised will result
    in an infinite loop to occurs stack overflow (and SIGSEGV).

commit da7d7f881b
Author: dearblue <dearblue@users.noreply.github.com>
Date:   Sun Jan 20 12:00:38 2019 +0900

    Fix NULL pointer dereference `mrb->nomem_err` when not initialized

    Add internal functions (not `static`):

      * `mrb_raise_nomemory()`
      * `mrb_core_init_abort()`

src/proc.c

@@ -8,6 +8,7 @@
 #include <mruby/class.h>
 #include <mruby/proc.h>
 #include <mruby/opcode.h>
+#include <mruby/data.h>
 
 static const mrb_code call_iseq[] = {
   OP_CALL,
@@ -122,7 +123,14 @@ mrb_proc_new_cfunc_with_env(mrb_state *mrb, mrb_func_t func, mrb_int argc, const
   p->flags |= MRB_PROC_ENVSET;
   mrb_field_write_barrier(mrb, (struct RBasic*)p, (struct RBasic*)e);
   MRB_ENV_UNSHARE_STACK(e);
+
+  /* NOTE: Prevents keeping invalid addresses when NoMemoryError is raised from `mrb_malloc()`. */
+  e->stack = NULL;
+  MRB_ENV_SET_STACK_LEN(e, 0);
+
   e->stack = (mrb_value*)mrb_malloc(mrb, sizeof(mrb_value) * argc);
+  MRB_ENV_SET_STACK_LEN(e, argc);
+
   if (argv) {
     for (i = 0; i < argc; ++i) {
       e->stack[i] = argv[i];
@@ -286,14 +294,25 @@ mrb_proc_arity(const struct RProc *p)
   return arity;
 }
 
+static void
+tempirep_free(mrb_state *mrb, void *p)
+{
+  if (p) mrb_irep_free(mrb, (mrb_irep *)p);
+}
+
+static const mrb_data_type tempirep_type = { "temporary irep", tempirep_free };
+
 void
 mrb_init_proc(mrb_state *mrb)
 {
   struct RProc *p;
   mrb_method_t m;
-  mrb_irep *call_irep = (mrb_irep *)mrb_malloc(mrb, sizeof(mrb_irep));
+  struct RData *irep_obj = mrb_data_object_alloc(mrb, mrb->object_class, NULL, &tempirep_type);
+  mrb_irep *call_irep;
   static const mrb_irep mrb_irep_zero = { 0 };
 
+  call_irep = (mrb_irep *)mrb_malloc(mrb, sizeof(mrb_irep));
+  irep_obj->data = call_irep;
   *call_irep = mrb_irep_zero;
   call_irep->flags = MRB_ISEQ_NO_FREE;
   call_irep->iseq = call_iseq;
@@ -305,6 +324,7 @@ mrb_init_proc(mrb_state *mrb)
   mrb_define_method(mrb, mrb->proc_class, "arity", proc_arity, MRB_ARGS_NONE());
 
   p = mrb_proc_new(mrb, call_irep);
+  irep_obj->data = NULL;
   MRB_METHOD_FROM_PROC(m, p);
   mrb_define_method_raw(mrb, mrb->proc_class, mrb_intern_lit(mrb, "call"), m);
   mrb_define_method_raw(mrb, mrb->proc_class, mrb_intern_lit(mrb, "[]"), m);