Fix memory leak when deallocating prepared statement in postgres_fdw

The leak is minor, so no backpatch is done.  Oversight in 21734d2f.

Reported-by: Tom Lane

contrib/postgres_fdw/postgres_fdw.c

@@ -4063,6 +4063,7 @@ deallocate_query(PgFdwModifyState *fmstate)
 	if (PQresultStatus(res) != PGRES_COMMAND_OK)
 		pgfdw_report_error(ERROR, res, fmstate->conn, true, sql);
 	PQclear(res);
+	pfree(fmstate->p_name);
 	fmstate->p_name = NULL;
 }