Commit 2da221e217 added a missing abort
if datalen < 0 check on a code path and an assertion inside
iax_frame_wrap if we ever encountered a frame with a negative frame
length (which will eventually cause a crash).
Add another missing abort check for negative datalen, exposed by this
assertion. (Similar to the previous commit, this is a video frame with
a datalen of -1).
Resolves: #1865
Naveen Albert
ae95ec0591