Fixes: Division by 0
Fixes: -copyts -start_at_zero -itsoffset 00:00:01 -itsscale 1 -ss 00:00:02  -i zgclab/ffmpeg_crash/poc1 output.mp4

Found-by: Wang Dawei and Zhou Geng, from Zhongguancun Laboratory
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>