div by zero aspect fix

Originally committed as revision 3850 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -3469,6 +3469,8 @@ static int decode_slice_header(H264Context *h){
         s->avctx->width = s->width;
         s->avctx->height = s->height;
         s->avctx->sample_aspect_ratio= h->sps.sar;
+        if(!s->avctx->sample_aspect_ratio.den)
+            s->avctx->sample_aspect_ratio.den = 1;
 
         if(h->sps.timing_info_present_flag && h->sps.fixed_frame_rate_flag){
             s->avctx->frame_rate = h->sps.time_scale;