pgsql: Don't access catalogs to validate GUCs when not connected to aD

Don't access catalogs to validate GUCs when not connected to a DB.

Vignesh found this bug in the check function for
default_table_access_method's check hook, but that was just copied
from older GUCs. Investigation by Michael and me then found the bug in
further places.

When not connected to a database (e.g. in a walsender connection), we
cannot perform (most) GUC checks that need database access. Even when
only shared tables are needed, unless they're
nailed (c.f. RelationCacheInitializePhase2()), they cannot be accessed
without pg_class etc. being present.

Fix by extending the existing IsTransactionState() checks to also
check for MyDatabaseOid.

Reported-By: Vignesh C, Michael Paquier, Andres Freund
Author: Vignesh C, Andres Freund
Discussion: https://postgr.es/m/CALDaNm1KXK9gbZfY-p_peRFm_XrBh1OwQO1Kk6Gig0c0fVZ2uw%40mail.gmail.com
Backpatch: 9.4-

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b8607e15e0fa0a3ac20779e39eea24fd67ebb650

Modified Files
--------------
src/backend/commands/tablespace.c  | 14 ++++++++------
src/backend/utils/cache/ts_cache.c |  8 +++++---
2 files changed, 13 insertions(+), 9 deletions(-)