res_pjsip_registrar.c: Miscellaneous cleanup in rx_task().

* Fix variable shadowing of 'updated' by renaming it to 'contact_update'.

* Checked 'contact_update' for ast_sorcery_copy() failure.

* Removed silly use of RAII_VAR() for 'contact_update'.
........

Merged revisions 410995 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410996 65c4cc65-6c06-0410-ace0-fbb531ad65f3