Fixes a segfault occuring when performing attended transfer with FAXOPT(gateway)=yes

(closes issue ASTERISK-19184)
Reported by: Alexandr
........

Merged revisions 353962 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@353963 65c4cc65-6c06-0410-ace0-fbb531ad65f3