Prevent segfault for interpolated iLBC frames

When iLBC is being used with a jitter buffer and the jb has to
interpolate frames, it generates frames with a null pointer and a
non-zero datalen. This is now handled properly.

(closes issue ASTERISK-20914)
Reported By: John McEleney
Patches:
  ASTERISK-20914-1.8.diff uploaded by Matt Jordan (license 6283)
........

Merged revisions 379718 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@379719 65c4cc65-6c06-0410-ace0-fbb531ad65f3