Prevent a crash in res_pjsip_dtmf_info.c

This change makes sure that a content type header exists before
checking the contents of the header against known SIP INFO DTMF content
types.
........

Merged revisions 398206 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@398207 65c4cc65-6c06-0410-ace0-fbb531ad65f3