res_pjsip_registrar_expire.c: Cleanup scheduler leaks on unload/shutdown.

res_pjsip_registrar_expire.c: Cleanup scheduler leaks on unload/shutdown.

Contact expiration object refs were leaked when the module was unloaded.

* Made empty the scheduler of entries before destroying it to release the
object ref held by the scheduler entry.

Review: https://reviewboard.asterisk.org/r/4523/
........

Merged revisions 433596 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/certified/branches/13.1@433618 65c4cc65-6c06-0410-ace0-fbb531ad65f3