Improve pg_clear_extended_stats() with incorrect relation/stats combinationHEADmaster

Issue fat-fingered in d756fa1019ff, noticed while doing more review of
the main patch set proposed.  I have missed the fact that this can be
triggered by specifying an extended stats object that does not match
with the relation specified and already locked.  Like the cases where
an object defined in input is missing, the code is changed to issue a
WARNING instead of a confusing cache lookup failure.

A regression test is added to cover this case.

Discussion: https://postgr.es/m/CADkLM=dpz3KFnqP-dgJ-zvRvtjsa8UZv8wDAQdqho=qN3kX0Zg@mail.gmail.com

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