Fix nodeTidscan.c to not trigger an error if the block number portion of

a user-supplied TID is out of range for the relation.  This is needed to
preserve compatibility with our pre-8.3 behavior, and it is sensible anyway
since if the query were implemented by brute force rather than optimized
into a TidScan, the behavior for a non-existent TID would be zero rows out,
never an error.  Per gripe from Gurjeet Singh.