res_fax: Resolve module reference leak caused by reserved sessions

Remove reference to module providing reserved session after
adding a reference to the final module.  This re-reference
is done to ensure that module references are correct even
if the final session selects a different module than the
reserved session.

ASTERISK-18923 #close
Reported by: Grigoriy Puzankin
Review: https://reviewboard.asterisk.org/r/4048/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@425405 f38db490-d61c-443f-a65b-d21fe96a405b