Bug 18457 - process_message_queue.pl will die if a patron has no sms_provider_id...

Bug 18457 - process_message_queue.pl will die if a patron has no sms_provider_id set but sms via email is enabled for that patron

If SMS via Email is enabled, and a patron has opted for SMS messages, but has not selected a service provider, the cronjob will die with the error
Can't call method "domain" on an undefined value at /usr/share/koha/lib/C4/Letters.pm line 1055.
This will cause all messages that come after the error to not be sent!

Test Plan:
1) Enable SMS via Email
2) Enable SMS for a patron, but don't set a provider
3) Perform an action that will trigger an sms message to go into
   the holds queue ( item due, item checkout, etc )
4) Run process_message_queue.pl, note the error
5) Apply the patch
4) Run process_message_queue.pl, no error this time!

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>