fix single tuple result memory leak (#6724)

We should not omit to free PGResult when we receive single tuple result
from an internal backend.
Single tuple results are normally freed by our ReceiveResults for
`tupleDescriptor != NULL` flow but not for those with `tupleDescriptor
== NULL`. See PR #6722 for details.

DESCRIPTION: Fixes memory leak issue with query results that returns
single row.

(cherry picked from commit 9e69dd0)

src/backend/distributed/executor/adaptive_executor.c

@@ -3930,6 +3930,7 @@ ReceiveResults(WorkerSession *session, bool storeRows)
 		TupleDesc tupleDescriptor = tupleDest->tupleDescForQuery(tupleDest, queryIndex);
 		if (tupleDescriptor == NULL)
 		{
+			PQclear(result);
 			continue;
 		}