staging: vchiq_core: Bail out if service is NULL

In the unlikely case that service is NULL we should bail out instead
of calling BUG_ON(). The other BUG_ON calls will be fixed in separate
patches.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>