settings: check bt pan service before use it

If pan service is not allowed, the service listener is notified, but with an invalid reference.
This patch checks that the pan service has a valid reference before use it.

Change-Id: I3fed496d4a6b3c8d2620bc5913943e58fecddf6d
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>