Fix a crash that can result if text codecs are allowed but textsupport is disabled.

(closes issue #15596)
Reported by: fabled
Patches:
      sip-red.patch uploaded by fabled (license 448)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@209516 65c4cc65-6c06-0410-ace0-fbb531ad65f3