avcodec/mpeg4videodec: set field durations to safe values when they are invalid

Fixes division by 0
Fixes: 3c980274cecaca6ca5d6e517c0c1983e-signal_sigabrt_7ffff7125425_410_cov_3621042467_2_audio_streams.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
