res_pjsip_outbound_registration: Ensure URI validation happens in a pjlib thread.

This change moves outbound registration URI validation into the task executed
within a pjlib thread.

Reported by: Andrew Nagy
........

Merged revisions 404725 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@404726 65c4cc65-6c06-0410-ace0-fbb531ad65f3