avformat/utils: Check AVFormatContext->duration in compute_chapters_end() before use

Fixes integer overflow
Fixes: 266ee543812e934f7b4a72923a2701d4/signal_sigabrt_7ffff6ae7cc9_7322_85218d61759d461bdf7387180e8000c9.ogg

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
