set stream aspect-ratio to video aspect-ratio, as ffmpeg checks that

from r15367 onward and refuses to mux without them matching.

Should fix flv-muxing.
(cherry picked from commit 0a778174)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/avformat/mux.c

@@ -252,6 +252,8 @@ static int AddStream( sout_mux_t *p_mux, sout_input_t *p_input )
                    &codec->sample_aspect_ratio.den,
                    i_aspect_num * (int64_t)codec->height,
                    i_aspect_den * (int64_t)codec->width, 1 << 30 );
+        stream->sample_aspect_ratio.num = codec->sample_aspect_ratio.num;
+        stream->sample_aspect_ratio.den = codec->sample_aspect_ratio.den;
         codec->time_base.den = p_input->p_fmt->video.i_frame_rate;
         codec->time_base.num = p_input->p_fmt->video.i_frame_rate_base;
         break;