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 be handled properly.

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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@379718 f38db490-d61c-443f-a65b-d21fe96a405b