pgsql: Fix two crashes on a concurrently dropped schema

Fix two crashes on a concurrently dropped schema

pg_identify_object() looks up the object's schema name without a lock
on the schema, so a concurrent DROP SCHEMA can make get_namespace_name()
return NULL, which was then passed to quote_identifier(). Report the
schema as NULL instead, as is already done for objects that have gone
away.

Similarly, schema_to_xml() and the related SQL/XML functions look up
the schema name without a lock on the schema, and a concurrent DROP
SCHEMA can make get_namespace_name() return NULL, which was then
dereferenced. Report that the schema does not exist, or skip it when
mapping all schemas of the database.

Author: Baji Shaik <baji(dot)pgdev(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/CA+fm-RPD0m9QL8W+804SGBBxn5iFz5wn=kncaP9SkM-s4PUANw@mail.gmail.com
Backpatch-through: 14

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/77705d1e5a616339c3e92f586bbfa7b6bb562beb

Modified Files
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src/backend/catalog/objectaddress.c | 13 +++++++-----
src/backend/utils/adt/xml.c | 40 ++++++++++++++++++++++++++++++-------
2 files changed, 41 insertions(+), 12 deletions(-)