Fix wrong mrb_bug use found with mruby-clang-plugin.

src/parse.y

@@ -3323,7 +3323,7 @@ backref_error(parser_state *p, node *n)
     yyerror_i(p, "can't set variable $%c", (int)(intptr_t)n->cdr);
   }
   else {
-    mrb_bug(p->mrb, "Internal error in backref_error() : n=>car == %d", c);
+    mrb_bug(p->mrb, "Internal error in backref_error() : n=>car == %S", mrb_fixnum_value(c));
   }
 }