mp4: Don't abort on truncated files.

If the source file got truncated it may happen that a chunk cannot
be read because the computed file offset is beyond EOF. Currently,
aac_afh_get_chunk() aborts in this case because we assert that the
file offset is within range. Return a proper error code instead and
also change aac_get_file_info() to bail out if aac_afh_get_chunk()
returns negative.