avformat/oggparsetheora: Check duration to be not AV_NOPTS_VALUE (and positive) before use

Fixes: 1536b9b096a8f95b742bae9d3d761cc6/signal_sigsegv_294aaed_7322_4fad88a38dc8952dc20dcb60c1895758.ogg

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