ffmpeg.c: transcode_video(): do not go through filter network if encoding is not required

This fixes a segmentation fault when doing a transcoding and a stream
copy of the same input stream at the same time, e.g.:

ffmpeg -i in.mkv -c:v mpeg2video transcode.m2v -c copy copy.ts

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>