avformat/pjsdec: dont increase pointer when its already at the end in read_ts()

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f91f2de7764_2649_PJS_capability_tester.pjs
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
