Fixes a regression in ca2f19b9 with some mov/mp4 files. The files have
several NAL units in the supposed single NAL unit after the size field.
Annex B start code prefixes are used to separate them. The first NAL unit
is correctly parsed but the buffer does not point to the next size field.
Instead semi random data (it seems to be the rbsp_stop_one_bit and the
start code prefix) is then parsed as length and will exceed the
remaining length of the buffer.

Patch based on the code in h264's decode_nal_units() and a similar
patch by Hendrik Leppkes in FFmpeg (a9bb4cf8).

Bug-Id: ffmpeg/trac5529
Reported-By: Vittorio Giovara