pgsql: Fix nodeTidscan.c to not trigger an error if the block number

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/executor:
        nodeTidscan.c (r1.58 -> r1.59)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeTidscan.c?r1=1.58&r2=1.59)