fixed error-handling for mrb_open().

When mrb_open() is called again, it is not checked.

mrbgems/mruby-bin-debugger/tools/mrdb/mrdb.c

@@ -651,13 +651,13 @@ main(int argc, char **argv)
   mrb_debug_context* dbg_backup;
   debug_command *cmd;
 
+ l_restart:
+
   if (mrb == NULL) {
     fputs("Invalid mrb_state, exiting mruby\n", stderr);
     return EXIT_FAILURE;
   }
 
- l_restart:
-
   /* parse command parameters */
   n = parse_args(mrb, argc, argv, &args);
   if (n == EXIT_FAILURE || args.rfp == NULL) {