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-

src/backend/access/table/tableamapi.c

@@ -18,6 +18,7 @@
 #include "catalog/pg_am.h"
 #include "catalog/pg_proc.h"
 #include "commands/defrem.h"
+#include "miscadmin.h"
 #include "utils/fmgroids.h"
 #include "utils/memutils.h"
 #include "utils/syscache.h"
@@ -119,10 +120,11 @@ check_default_table_access_method(char **newval, void **extra, GucSource source)
 	}
 
 	/*
-	 * If we aren't inside a transaction, we cannot do database access so
-	 * cannot verify the name.  Must accept the value on faith.
+	 * If we aren't inside a transaction, or not connected to a database, we
+	 * cannot do the catalog access necessary to verify the method.  Must
+	 * accept the value on faith.
 	 */
-	if (IsTransactionState())
+	if (IsTransactionState() && MyDatabaseId != InvalidOid)
 	{
 		if (!OidIsValid(get_table_am_oid(*newval, true)))
 		{