pgsql: Check if ii_AmCache is NULL in aminsertcleanup

Check if ii_AmCache is NULL in aminsertcleanup

Fix a bug introduced by c1ec02be1d79. It may happen that the executor
opens indexes on the result relation, but no rows end up being inserted.
Then the index_insert_cleanup still gets executed, but passes down NULL
to the AM callback. The AM callback may not expect this, as is the case
of brininsertcleanup, leading to a crash.

Fixed by only calling the cleanup callback if (ii_AmCache != NULL). This
way the AM can simply assume to only see a valid cache.

Reported-by: Richard Guo
Discussion: https://postgr.es/m/CAMbWs4-w9qC-o9hQox9UHvdVZAYTp8OrPQOKtwbvzWaRejTT=Q@mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/a82ee7ef3aacc2073d3ef5f4b7e5067fa08ea76c

Modified Files
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src/backend/access/index/indexam.c | 2 +-
src/test/regress/expected/brin.out | 6 ++++++
src/test/regress/sql/brin.sql      | 7 +++++++
3 files changed, 14 insertions(+), 1 deletion(-)