pgsql: pgstattuple: Fix failure with pgstathashindex() for partitioned

pgstattuple: Fix failure with pgstathashindex() for partitioned indexes

As coded, the function relied on index_open() when opening an index
relation, allowing partitioned indexes to be processed by
pgstathashindex(). This was leading to a "could not open file" error
because partitioned indexes have no physical files, or to a crash with
an assertion failure (like on HEAD).

This issue is fixed by applying the same checks as the other stat
functions for indexes, with a lookup at both RELKIND_INDEX and the index
AM expected.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/18246-f4d9ff7cb3af77e6@postgresql.org
Backpatch-through: 12

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b745f168042e253e92a71bb4d4554789d72c21c5

Modified Files
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contrib/pgstattuple/expected/pgstattuple.out | 11 +++++++----
contrib/pgstattuple/pgstatindex.c | 5 ++---
contrib/pgstattuple/sql/pgstattuple.sql | 2 ++
3 files changed, 11 insertions(+), 7 deletions(-)