Fix error handlings for mrb_open_core().

tasks/mrbgem_spec.rake

@@ -183,6 +183,7 @@ module MRuby
 
       def print_gem_test_header(f)
         print_gem_comment(f)
+        f.puts %Q[#include <stdio.h>]
         f.puts %Q[#include <stdlib.h>]
         f.puts %Q[#include "mruby.h"]
         f.puts %Q[#include "mruby/irep.h"]

tasks/mrbgems_test.rake

@@ -47,6 +47,10 @@ MRuby.each_target do
           g.test_rbfiles.count.times do |i|
             f.puts %Q[  ai = mrb_gc_arena_save(mrb);]
             f.puts %Q[  mrb2 = mrb_open_core(mrb_default_allocf, NULL);]
+            f.puts %Q[  if (mrb2 == NULL) {]
+            f.puts %Q[    fprintf(stderr, "Invalid mrb_state, exiting \%s", __FUNCTION__);]
+            f.puts %Q[    exit(EXIT_FAILURE);]
+            f.puts %Q[  }]
             dep_list.each do |d|
               f.puts %Q[  GENERATED_TMP_mrb_#{d.funcname}_gem_init(mrb2);]
               f.puts %Q[  mrb_state_atexit(mrb2, GENERATED_TMP_mrb_#{d.funcname}_gem_final);]

test/init_mrbtest.c

@@ -18,6 +18,10 @@ mrb_init_mrbtest(mrb_state *mrb)
   mrb_load_irep(mrb, mrbtest_assert_irep);
 
   core_test = mrb_open_core(mrb_default_allocf, NULL);
+  if (core_test == NULL) {
+    fprintf(stderr, "Invalid mrb_state, exiting %s", __FUNCTION__);
+    exit(EXIT_FAILURE);
+  }
   mrb_init_test_driver(core_test, mrb_test(mrb_gv_get(mrb, mrb_intern_lit(mrb, "$mrbtest_verbose"))));
   mrb_load_irep(core_test, mrbtest_assert_irep);
   mrb_load_irep(core_test, mrbtest_irep);