fftools/ffmpeg: Check if we do have also a filter_complex filter.

Right now, the code check for no filter description, but if we use a
filter_complex, the code will use the AVFrame.duration which could be
wrong in case of using fps filter.

How to reproduce the problem:
ffmpeg -f lavfi -i testsrc=duration=1 -vf fps=fps=50 -vsync 1 -f null -
output 50 frames

ffmpeg -f lavfi -i testsrc=duration=1 -filter_complex fps=fps=50 -vsync 1 -f null -
output 51 frames

With this commit, the same command will always output 50 frames.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

fftools/ffmpeg.c

@@ -1079,6 +1079,7 @@ static void do_video_out(OutputFile *of,
 
     if (!ost->filters_script &&
         !ost->filters &&
+        (nb_filtergraphs == 0 || !filtergraphs[0]->graph_desc) &&
         next_picture &&
         ist &&
         lrintf(next_picture->pkt_duration * av_q2d(ist->st->time_base) / av_q2d(enc->time_base)) > 0) {