res_fax: Fix reference leak caused by gateway sessions

Fax gateway session objects can be re-used, causing the
same gateway session to be added to faxregistry.container
more than once.  This change causes fax_session_new to
remove the reserved session from the container before
it's id is changed, ensuring it's possible for the
session to be freed.

ASTERISK-24392 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/4049/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@425457 65c4cc65-6c06-0410-ace0-fbb531ad65f3