postgres_fdw and dblink should check if backend has MyProcPort

postgres_fdw and dblink should check if backend has MyProcPort

before checking ->has_scram_keys.  MyProcPort is NULL in background
workers.  So this could crash for example if a background worker
accessed a suitable configured foreign table.

Author: Alexander Pyhalov <a.pyhalov@postgrespro.ru>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/27b29a35-9b96-46a9-bc1a-914140869dac%40gmail.com