avformat/pva: Make sure the header is large enough before reading the timestamp from it

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f34b5dc6d58_2674_PVA_test-partial.pva
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
