chan_iax2: Add another check to abort frame handling if datalen < 0.

chan_iax2: Add another check to abort frame handling if datalen < 0.

Commit 2da221e217cbff957af928e8df43ee25583232d1 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