avformat/movenc: Check frame rate before use.

Fixes division by 0
This is similar to how avg_frame_rate is checked elsewhere
Fixes: 6d24add0455f41b1b45b7ba615cd46f3/asan_generic_dc34c3_5480_0a2ef411cae999b9871ed71a2e481b71.mov

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