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/error.c

@@ -185,6 +185,16 @@ mrb_exc_set(mrb_state *mrb, mrb_value exc)
   }
 }
 
+static mrb_noreturn void
+exc_throw(mrb_state *mrb, mrb_value exc)
+{
+  if (!mrb->jmp) {
+    mrb_p(mrb, exc);
+    abort();
+  }
+  MRB_THROW(mrb->jmp);
+}
+
 MRB_API mrb_noreturn void
 mrb_exc_raise(mrb_state *mrb, mrb_value exc)
 {
@@ -197,11 +207,7 @@ mrb_exc_raise(mrb_state *mrb, mrb_value exc)
     }
     mrb_exc_set(mrb, exc);
   }
-  if (!mrb->jmp) {
-    mrb_p(mrb, exc);
-    abort();
-  }
-  MRB_THROW(mrb->jmp);
+  exc_throw(mrb, exc);
 }
 
 MRB_API mrb_noreturn void
@@ -550,6 +556,52 @@ mrb_argnum_error(mrb_state *mrb, mrb_int argc, int min, int max)
 #undef FMT
 }
 
+void mrb_core_init_printabort(void);
+
+int
+mrb_core_init_protect(mrb_state *mrb, void (*body)(mrb_state *, void *), void *opaque)
+{
+  struct mrb_jmpbuf *prev_jmp = mrb->jmp;
+  struct mrb_jmpbuf c_jmp;
+  int err = 1;
+
+  MRB_TRY(&c_jmp) {
+    mrb->jmp = &c_jmp;
+    body(mrb, opaque);
+    err = 0;
+  } MRB_CATCH(&c_jmp) {
+    if (mrb->exc) {
+      mrb_p(mrb, mrb_obj_value(mrb->exc));
+      mrb->exc = NULL;
+    }
+    else {
+      mrb_core_init_printabort();
+    }
+  } MRB_END_EXC(&c_jmp);
+
+  mrb->jmp = prev_jmp;
+
+  return err;
+}
+
+mrb_noreturn void
+mrb_core_init_abort(mrb_state *mrb)
+{
+  mrb->exc = NULL;
+  exc_throw(mrb, mrb_nil_value());
+}
+
+mrb_noreturn void
+mrb_raise_nomemory(mrb_state *mrb)
+{
+  if (mrb->nomem_err) {
+    mrb_exc_raise(mrb, mrb_obj_value(mrb->nomem_err));
+  }
+  else {
+    mrb_core_init_abort(mrb);
+  }
+}
+
 void
 mrb_init_exception(mrb_state *mrb)
 {