Assertion on create index concurrently

I found one annoying assertion in the CREATE INDEX CONCURRENTLY
operation. It will happen if you add into WHERE part of CREATE INDEX
CONCURRENTLY some function that will make transactional update (see an
example in the attached patch).

It looks like a hack, but it lead to assertion and restart of the
instance, that isn't good.
I suggest to perform error, not assertion in this case (see the patch).

This problem affects PG 9.6 - 13. The master branch was fixed 9 months
ago by commit 83158f7, that made this operation transactional, but still.

--
regards,
Andrey Lepikhov
Postgres Professional