Put back gistgettuple's check for backwards scan request.

On reflection it's a bad idea for the KNNGIST patch to have removed that.
We don't want it silently returning incorrect answers.

src/backend/access/gist/gistget.c

@@ -459,8 +459,12 @@ Datum
 gistgettuple(PG_FUNCTION_ARGS)
 {
 	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
+	ScanDirection dir = (ScanDirection) PG_GETARG_INT32(1);
 	GISTScanOpaque so = (GISTScanOpaque) scan->opaque;
 
+	if (dir != ForwardScanDirection)
+		elog(ERROR, "GiST only supports forward scan direction");
+
 	if (!so->qual_ok)
 		PG_RETURN_BOOL(false);