huffyuvdec: Skip len==0 cases

      Fixes vlc decoding for hypothetical files that would contain such cases.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 0dfc01c2bbf4b71bb56201bc4a393321e15d1b31)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>