Fixes assertion failure

Fixes: f12c79769501ee471a5c731b8304ffcb/signal_sigabrt_7ffff6ac8cc9_3766_cov_4267830138_undemuxable_h264.ts

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>