h261: check mtype.

Fixes out of array read

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit ec3cd74f2dab8e3e8234ccb994132b23d3098585)
1 file changed