pgsql: Avoid crash in rare case of concurrent DROP

Avoid crash in rare case of concurrent DROP

When a role being dropped contains is referenced by catalog objects that
are concurrently also being dropped, a crash can result while trying to
construct the string that describes the objects.  Suppress that by
ignoring objects whose descriptions are returned as NULL.

The majority of relevant codesites were already cautious about this
already; we had just missed a couple.

This is an old bug, so backpatch all the way back.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/17126-21887f04508cb5c8@postgresql.org

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e0eaeafd66462b1a584831f35f10ec1e017c3845

Modified Files
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src/backend/catalog/dependency.c  | 31 ++++++++++++++++++++-----------
src/backend/catalog/pg_shdepend.c |  6 ++++++
2 files changed, 26 insertions(+), 11 deletions(-)