avformat/pva: Make sure the first byte of pes_header_data has been initialized

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f53c1d0e95c_2674_PVA_test-partial.pva
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
