lavu/frame: fix malloc error path in av_frame_copy_props()

The error path frees all side data, but forgets to reset the side data
count. This can blow up later in av_frame_unref() and free_side_data().

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit a400edbb6d)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/frame.c

@@ -489,6 +489,7 @@ int av_frame_copy_props(AVFrame *dst, const AVFrame *src)
                 av_dict_free(&dst->side_data[i]->metadata);
             }
             av_freep(&dst->side_data);
+            dst->nb_side_data = 0;
             return AVERROR(ENOMEM);
         }
         memcpy(sd_dst->data, sd_src->data, sd_src->size);