ffplay: in input_config_props(), honour the SAR specified in the codec context

Use the value specified in the codec context for setting the
filterchain sample aspect ratio, when it is not specified in the
stream context.

Consistent with the ffmpeg behavior.

Fix trac issue #398.

ffplay.c

@@ -1641,10 +1641,12 @@ static int input_config_props(AVFilterLink *link)
 {
     FilterPriv *priv  = link->src->priv;
     AVCodecContext *c = priv->is->video_st->codec;
+    AVStream *s = priv->is->video_st;
 
     link->w = c->width;
     link->h = c->height;
-    link->sample_aspect_ratio = priv->is->video_st->sample_aspect_ratio;
+    link->sample_aspect_ratio = s->sample_aspect_ratio.num ?
+        s->sample_aspect_ratio : c->sample_aspect_ratio;
     link->time_base = priv->is->video_st->time_base;
 
     return 0;