Check mrb_open return value for NULL

src/parse.y

@@ -5518,6 +5518,11 @@ main()
   mrb_state *mrb = mrb_open();
   int n;
 
+  if (mrb == NULL) {
+    fprintf(stderr, "Invalid mrb_state, exiting parser test");
+    return EXIT_FAILURE;
+  }
+
   n = mrb_compile_string(mrb, "\
 def fib(n)\n\
   if n<2\n\