use mrb_str_buf_new to create a buffer to be used by mrb_str_buf_cat

src/vm.c

@@ -468,7 +468,8 @@ localjump_error(mrb_state *mrb, localjump_error_kind kind)
   mrb_value msg;
   mrb_value exc;
 
-  msg = mrb_str_new(mrb, lead, sizeof(lead) - 1);
+  msg = mrb_str_buf_new(mrb, sizeof(lead) + 7);
+  mrb_str_buf_cat(mrb, msg, lead, sizeof(lead) - 1);
   mrb_str_buf_cat(mrb, msg, kind_str[kind], kind_str_len[kind]);
   exc = mrb_exc_new3(mrb, E_LOCALJUMP_ERROR, msg);
   mrb->exc = (struct RObject*)mrb_object(exc);