avfilter/video: pick sar from link

It should not be needed for each filter that sets sample aspect ratio
to set it explicitly also for each and every frame, instead that is
automatically done in get_buffer call.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/video.c

@@ -43,6 +43,7 @@ AVFrame *ff_null_get_video_buffer(AVFilterLink *link, int w, int h)
 
 AVFrame *ff_default_get_video_buffer(AVFilterLink *link, int w, int h)
 {
+    AVFrame *frame = NULL;
     int pool_width = 0;
     int pool_height = 0;
     int pool_align = 0;
@@ -86,7 +87,13 @@ AVFrame *ff_default_get_video_buffer(AVFilterLink *link, int w, int h)
         }
     }
 
-    return ff_frame_pool_get(link->frame_pool);
+    frame = ff_frame_pool_get(link->frame_pool);
+    if (!frame)
+        return NULL;
+
+    frame->sample_aspect_ratio = link->sample_aspect_ratio;
+
+    return frame;
 }
 
 AVFrame *ff_get_video_buffer(AVFilterLink *link, int w, int h)