MariaRocks port: Make SQL layer allow errors in start_consistent_snapshot().

rocksdb.cons_snapshot_read_committed test used to crash.
This happened, because
- MyRocks produces an error when one attempts to do a START TRANSACTION
  WITH CONSISTENT SNAPSHOT with @@tx_isolation=REPEATABLE_READ
- MariaDB's SQL layer didn't assume that errors are possible inside
  hton->start_constistent_snapshot() call.

sql/sql_parse.cc

@@ -5428,7 +5428,9 @@ mysql_execute_command(THD *thd)
                   (longlong) thd->thread_id);
       goto error;
     }
-    my_ok(thd);
+    /* MyRocks: hton->start_consistent_snapshot call may fail with an error */
+    if (!thd->is_error())
+      my_ok(thd);
     break;
   case SQLCOM_COMMIT:
   {