libavfilter/vf_cover_rect.c: free the allocated frame

Signed-off-by: Limin Wang <lance.lmwang@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_cover_rect.c

@@ -198,6 +198,7 @@ static av_cold void uninit(AVFilterContext *ctx)
 
     if (cover->cover_frame)
         av_freep(&cover->cover_frame->data[0]);
+    av_frame_free(&cover->cover_frame);
 }
 
 static av_cold int init(AVFilterContext *ctx)