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(at)gmail(dot)com>
Discussion: https://postgr.es/m/17126-21887f04508cb5c8@postgresql.org

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/92224e470c830e73636de20689690f28037cd019

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