show error message when creating a table for matched records

When indexes are broken, there is a case that the value of
 matched_record_keys is null.
This commit shows error message for such a case.

ha_mroonga.cpp

@@ -8705,6 +8705,14 @@ FT_INFO *ha_mroonga::generic_ft_init_ext(uint flags, uint key_nr, String *key)
     matched_record_keys = grn_table_create(ctx, NULL, 0, NULL,
                                            GRN_OBJ_TABLE_HASH_KEY | GRN_OBJ_WITH_SUBREC,
                                            grn_table, 0);
+    if (!matched_record_keys) {
+      char error_message[MRN_MESSAGE_BUFFER_SIZE];
+      snprintf(error_message, MRN_MESSAGE_BUFFER_SIZE,
+               "failed to create a table for matched record: <%s>",
+               ctx->errbuf);
+      my_message(ER_ERROR_ON_READ, error_message, MYF(0));
+      GRN_LOG(ctx, GRN_LOG_ERROR, "%s", error_message);
+    }
   }
 
   grn_table_sort_key *sort_keys = NULL;