SCTE35 / CUEI Descriptor parsing bug. Descriptor lengths of one byte fail to parse, instead returning null.

The Spec from 2004 to 2016 shows the descriptor payload has never been
more than a single byte.
See SCTE 35 2016 Page 19 Table 2 (Section 8.2).

Correct the length validation check, and reject any descriptor that
IS NOT one byte long, as per the spec.
Signed-off-by: Jean-Paul Saman <jpsaman@videolan.org>
(cherry picked from commit 37f981c3)