avcodec/shorten: clear bitstream buffer

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f3ca95606fb_6393_luckynight-partial.shn
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
