Fix copy & paste mistake in pg_get_replication_slots().

XLogRecPtr was compared with InvalidTransactionId instead of
InvalidXLogRecPtr. As both are defined to the same value this doesn't
cause any actual problems, but it's still wrong.

Backpatch: 9.4-master, bug was introduced in 9.4

src/backend/replication/slotfuncs.c

@@ -268,7 +268,7 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 		else
 			nulls[i++] = true;
 
-		if (restart_lsn != InvalidTransactionId)
+		if (restart_lsn != InvalidXLogRecPtr)
 			values[i++] = LSNGetDatum(restart_lsn);
 		else
 			nulls[i++] = true;