Handle empty result set in libpqrcv_exec

Always return tupleslot and tupledesc from libpqrcv_exec.  This avoids
requiring callers to handle that separately.

Author: Petr Jelinek <petr.jelinek@2ndquadrant.com>
Reported-by: Michael Banck <michael.banck@credativ.de>

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -803,10 +803,6 @@ libpqrcv_processTuples(PGresult *pgres, WalRcvExecResult *walres,
 	MemoryContext	rowcontext;
 	MemoryContext	oldcontext;
 
-	/* No point in doing anything here if there were no tuples returned. */
-	if (PQntuples(pgres) == 0)
-		return;
-
 	/* Make sure we got expected number of fields. */
 	if (nfields != nRetTypes)
 		ereport(ERROR,
@@ -824,6 +820,10 @@ libpqrcv_processTuples(PGresult *pgres, WalRcvExecResult *walres,
 						   PQfname(pgres, coln), retTypes[coln], -1, 0);
 	attinmeta = TupleDescGetAttInMetadata(walres->tupledesc);
 
+	/* No point in doing more here if there were no tuples returned. */
+	if (PQntuples(pgres) == 0)
+		return;
+
 	/* Create temporary context for local allocations. */
 	rowcontext = AllocSetContextCreate(CurrentMemoryContext,
 									   "libpqrcv query result context",