avcodec/mpeg4videoenc: Use 64bit for time variables

Fixes assertion failure and integer overflow
Fixes: fc677bbea2c6f901763eb637b61fa5e2/signal_sigabrt_7ffff6ae7cb7_9556_dfd95f040a69f725d1b2f861bd491725.ivf

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