avcodec/mpegvideo_enc: fix memory leak

When the input frames contain side data, it will accumulate endlessly in
the coded frame, as av_frame_copy_props will append any new side data.

Fixes ticket #5799.

libavcodec/mpegvideo_enc.c

@@ -1735,6 +1735,7 @@ static void frame_end(MpegEncContext *s)
 
 #if FF_API_CODED_FRAME
 FF_DISABLE_DEPRECATION_WARNINGS
+    av_frame_unref(s->avctx->coded_frame);
     av_frame_copy_props(s->avctx->coded_frame, s->current_picture.f);
 FF_ENABLE_DEPRECATION_WARNINGS
 #endif