Check criticalSharedRelcachesBuilt in GetSharedSecurityLabel().

An extension may want to call GetSecurityLabel() on a shared object
before the shared relcaches are fully initialized. For instance, a
ClientAuthentication_hook might want to retrieve the security label on
a role.

Discussion: https://postgr.es/m/ecb7af0b26e3be1d96d291c8453a86f1f82d9061.camel@j-davis.com
Backpatch-through: 9.6

src/backend/commands/seclabel.c

@@ -243,8 +243,8 @@ GetSharedSecurityLabel(const ObjectAddress *object, const char *provider)
 
 	pg_shseclabel = table_open(SharedSecLabelRelationId, AccessShareLock);
 
-	scan = systable_beginscan(pg_shseclabel, SharedSecLabelObjectIndexId, true,
-							  NULL, 3, keys);
+	scan = systable_beginscan(pg_shseclabel, SharedSecLabelObjectIndexId,
+							  criticalSharedRelcachesBuilt, NULL, 3, keys);
 
 	tuple = systable_getnext(scan);
 	if (HeapTupleIsValid(tuple))