pgsql: Convert nullingrels match checks from Asserts to test-and-elog.

Convert nullingrels match checks from Asserts to test-and-elog.

It seems like the code that these checks are backstopping may have
a few bugs left in it.  Use a test-and-elog so that the tests are
performed even in non-assert builds, and so that we get something
more informative than "server closed the connection" on failure.

Committed separately with the idea that eventually we'll revert
this.  It might be awhile though.

Discussion: https://postgr.es/m/3014965.1684293045@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/867be9c0738bef591544d39985f886b7d8e99bf0

Modified Files
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src/backend/optimizer/plan/setrefs.c | 34 +++++++++++++++++++++-------------
1 file changed, 21 insertions(+), 13 deletions(-)