It might be impossible for this to cause a problem in non-debug builds,
since there'd be no opportunity for the relcache entry to get recycled
before the fetch.  It blows up nicely with -DRELCACHE_FORCE_RELEASE plus
valgrind, though.

Evidently introduced by careless refactoring in commit f0e44751.
Back-patch accordingly.

Discussion: https://postgr.es/m/27543.1528758304@sss.pgh.pa.us