Fix unsafe extraction of the OID part of a relation filename.

Fix unsafe extraction of the OID part of a relation filename.

Commit 8694cc96b did this randomly differently from other callers of
parse_filename_for_nontemp_relation().  Perhaps unsurprisingly,
the randomly different way is wrong; it fails to ensure the
extracted string is null-terminated.  Per buildfarm member skink.

Discussion: https://postgr.es/m/14453.1522001792@sss.pgh.pa.us