res_pjsip_outbound_registration: Fix reference leak.

res_pjsip_outbound_registration: Fix reference leak.

Every time a registration started, sip_outbound_registration_response_cb bumps
the ref count on client_state then pushes a handle_registration_response task.
handle_registration_response never unreffed it though.  So every time a
registration goes out, the ref count goes up by one.

This patch adds the unreffs to handle_registration_response.

Tested-by: George Joseph
Review: https://reviewboard.asterisk.org/r/4303/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@430395 65c4cc65-6c06-0410-ace0-fbb531ad65f3