Fix typo in tts_virtual_copyslot.

The code used the destination slot's natts where it intended to
use the source slot's natts.  Adding an Assert shows that there
is no case in "make check-world" where these counts are different,
so maybe this is a harmless bug, but it's still a bug.

Takayuki Tsunakawa

Discussion: https://postgr.es/m/0A3221C70F24FB45833433255569204D1FD34C0E@G01JPEXMBYT05

src/backend/executor/execTuples.c

@@ -244,7 +244,7 @@ tts_virtual_materialize(TupleTableSlot *slot)
 static void
 tts_virtual_copyslot(TupleTableSlot *dstslot, TupleTableSlot *srcslot)
 {
-	TupleDesc	srcdesc = dstslot->tts_tupleDescriptor;
+	TupleDesc	srcdesc = srcslot->tts_tupleDescriptor;
 
 	Assert(srcdesc->natts <= dstslot->tts_tupleDescriptor->natts);