Some of these are quite old, but that doesn't make them not bugs.
We'd rather report a failure via elog than SIGSEGV.

While at it, uniformly spell the error check as !RelationIsValid(rel)
rather than a bare rel == NULL test.  The machine code is the same
but it seems better to be consistent.

Coverity complained about this today, not sure why, because the
mistake is in fact old.