MDEV-10545: Server crashed in my_copy_fix_mb on querying I_S and P_S tables

After applying/replaying the transaction, the memory that
stored the query string was also wrongly freed.

sql/wsrep_applier.cc

@@ -365,8 +365,10 @@ wsrep_cb_status_t wsrep_commit_cb(void*         const     ctx,
   else
     rcode = wsrep_rollback(thd);
 
+  /* Cleanup */
   wsrep_set_apply_format(thd, NULL);
   thd->mdl_context.release_transactional_locks();
+  thd->reset_query();                           /* Mutex protected */
   free_root(thd->mem_root,MYF(MY_KEEP_PREALLOC));
   thd->tx_isolation= (enum_tx_isolation) thd->variables.tx_isolation;