avformat/matroskadec: Fix undefined shift in read_sint()

Fixes: asan_heap-oob_17212bc_2243_cov_594210248_h264_TTA.mkv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
