Fix bogus logic for reporting which hash partition conflicts.

Fix bogus logic for reporting which hash partition conflicts.

Commit efbfb6424 added logic for reporting exactly which existing
partition conflicts when complaining that a new hash partition's
modulus isn't compatible with the existing ones.  However, it
misunderstood the partitioning data structure, and would select
the wrong partition in some cases, or crash outright due to fetching
a bogus table OID in other cases.

Per bug #17076 from Alexander Lakhin.  Fix by Amit Langote;
some further work on the code comments by me.

Discussion: https://postgr.es/m/17076-89a16ae835d329b9@postgresql.org