Merge pull request #1416 from suzukaze/refactor-object-ext-object.c

I replace 0 with NULL because struct pointer be should set NULL in.

mrbgems/mruby-object-ext/src/object.c

@@ -80,7 +80,7 @@ mrb_obj_instance_exec(mrb_state *mrb, mrb_value self)
   case MRB_TT_SYMBOL:
   case MRB_TT_FIXNUM:
   case MRB_TT_FLOAT:
-    c = 0;
+    c = NULL;
     break;
   default:
     c = mrb_class_ptr(mrb_singleton_class(mrb, self));