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

@@ -240,6 +240,16 @@ 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)
 {
@@ -247,11 +257,7 @@ mrb_exc_raise(mrb_state *mrb, mrb_value exc)
     mrb_raise(mrb, E_TYPE_ERROR, "exception object expected");
   }
   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
@@ -483,6 +489,24 @@ mrb_no_method_error(mrb_state *mrb, mrb_sym id, mrb_value args, char const* fmt,
   mrb_exc_raise(mrb, exc);
 }
 
+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)
 {

src/gc.c

@@ -198,6 +198,8 @@ gettimeofday_time(void)
 
 #define objects(p) ((RVALUE *)p->objects)
 
+mrb_noreturn void mrb_raise_nomemory(mrb_state *mrb);
+
 MRB_API void*
 mrb_realloc_simple(mrb_state *mrb, void *p,  size_t len)
 {
@@ -221,12 +223,12 @@ mrb_realloc(mrb_state *mrb, void *p, size_t len)
   if (len == 0) return p2;
   if (p2 == NULL) {
     if (mrb->gc.out_of_memory) {
-      mrb_exc_raise(mrb, mrb_obj_value(mrb->nomem_err));
+      mrb_raise_nomemory(mrb);
       /* mrb_panic(mrb); */
     }
     else {
       mrb->gc.out_of_memory = TRUE;
-      mrb_exc_raise(mrb, mrb_obj_value(mrb->nomem_err));
+      mrb_raise_nomemory(mrb);
     }
   }
   else {