Fixes: null pointer dereference
Fixes: -stream_loop 1 -ss 00:00:05 -i zgclab/ffmpeg_crash/poc2 -codec:v copy -codec:a aac -y output.mp4

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