pgsql: Fix RelationIdGetRelation calls that weren't bothering witherro

Fix RelationIdGetRelation calls that weren't bothering with error checks.

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.

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5f73d7294d9fbaa36a0167e7aa6fa693c9c724e6

Modified Files
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src/backend/access/heap/heapam.c                | 4 ++++
src/backend/replication/logical/reorderbuffer.c | 6 +++++-
2 files changed, 9 insertions(+), 1 deletion(-)