Small improvement to tlist_matches_tupdesc()

I just found myself in execScan.c. I noticed that tlist_matches_tupdesc() is a bit wasteful in some cases as it goes to the trouble of matching the tlist to the TupleDesc item by item until it runs out of tlist items or finds a non-match. Once the loop completes it ensures that all tlist items were consumed, so basically in other words, it checks the lengths match.