avformat/psxstr: zero packet to prevent uninitialized data to leak through to the decoder

Fixes: msan_uninit-mem_7f150abf2e84_4817_descent-partial.str
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>