fixes crash during dtmf

During the processing of Cisco dtmf the dtmf samples were
not being calculated correctly.  In an attempt to determine
what sample rate was being used, a NULL frame was processed
which caused a crash.  This patch resolves this.

(closes issue #17248)
Reported by: falves11
Patches:
      issue_17248.diff uploaded by dvossel (license 671)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@264114 65c4cc65-6c06-0410-ace0-fbb531ad65f3