ffmpeg: check fclose return values

In the spirit of commit a956840cbc. Simple method to reproduce:
pass -vstats_file /dev/full to ffmpeg.

All raw fclose usages in ffmpeg.c taken care of here.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
