res_pjsip_outbound_registration: Create registration client in pj thread.

Depending on which threading was loading the outbound registration it was
possible for the registration client to be allocated outside of a pj thread.
This change moves the creation inside the synchronous task where it is
guaranteed it will occur in a pj thread.

Reported by: Rob Thomas
........

Merged revisions 404923 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@404924 f38db490-d61c-443f-a65b-d21fe96a405b