avformat/asf: clear uninitialized areas of packets before returning them

Fixes use of uninitialized variables
Fixes msan_uninit-mem_7f839282b6ce_7273_msn08_VBRq70_800x600.wmv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
