Fix missing PQclear() in libpqrcv_endstreaming().

This omission leaked one PGresult per WAL streaming cycle, which possibly
would never be enough to notice in the real world, but it's still a leak.

Per Coverity.  Back-patch to 9.3 where the error was introduced.

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -246,6 +246,7 @@ libpqrcv_endstreaming(TimeLineID *next_tli)
 		ereport(ERROR,
 				(errmsg("error reading result of streaming command: %s",
 						PQerrorMessage(streamConn))));
+	PQclear(res);
 
 	/* Verify that there are no more results */
 	res = PQgetResult(streamConn);