Avoid improbable PANIC during heap_update, redux.

Avoid improbable PANIC during heap_update, redux.

Commit 34f581c39 intended to ensure that RelationGetBufferForTuple
would acquire a visibility-map page pin in case the otherBuffer's
all-visible bit had become set since we last had lock on that page.
But I missed a case: when we're extending the relation, VM concerns
were dealt with only in the relatively-less-likely case that we
fail to conditionally lock the otherBuffer.  I think I'd believed
that we couldn't need to worry about it if the conditional lock
succeeds, which is true for the target buffer; but the otherBuffer
was unlocked for awhile so its bit might be set anyway.  So we need
to do the GetVisibilityMapPins dance, and then also recheck the
page's free space, in both cases.

Per report from Jaime Casanova.  Back-patch to v12 as the previous
patch was (although there's still no evidence that the bug is
reachable pre-v14).

Discussion: https://postgr.es/m/E1lWLjP-00006Y-Ml@gemulon.postgresql.org