wmaprodec: tighter check for num_vec_coeffs

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit b21ba20cc83c80fe56192fee3626a8087f37d806)

Conflicts:

	libavcodec/wmaprodec.c

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