avformat/swfdec: clear 4 bytes at the end of a packet if they are not initialized

Fixes use of uninitialized memory
Fixes part of msan_uninit-mem_7f055dd0ab1b_9558_videopop_guitar_300k.swf
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
