Free PQresult on error in pg_receivexlog.

The leak is fairly small and rare, but a leak nevertheless.

Per Coverity report. Backpatch to 9.2, where pg_receivexlog was added.
pg_basebackup shares the code, but it always exits on error, so there is
no real leak.

src/bin/pg_basebackup/receivelog.c

@@ -612,6 +612,7 @@ ReceiveXlogStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline,
 		fprintf(stderr,
 				_("%s: unexpected termination of replication stream: %s"),
 				progname, PQresultErrorMessage(res));
+		PQclear(res);
 		goto error;
 	}
 	PQclear(res);