Fix crash resulting from frames with invalid data pointers.

In ast_frdup the frame data union does not get set to point to malloced memory 
if the datalen is zero, so make sure to handle the same case in ast_frisolate
appropriately.

(closes issue #16058)
Reported by: atis
Patches: 
      bug16058-fix.patch uploaded by jpeeler (license 325)
Tested by: atis


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@243244 65c4cc65-6c06-0410-ace0-fbb531ad65f3