res_pjsip_outbound_registration.c: Minor code cleanup.

* Add an allocation failure check and assert in
sip_outbound_registration_response_cb().

* Made sip_outbound_registration_state_destroy() handle partially created
state objects from sip_outbound_registration_state_alloc().

Review: https://reviewboard.asterisk.org/r/4366/
........

Merged revisions 430957 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@430958 65c4cc65-6c06-0410-ace0-fbb531ad65f3