Fix memory leak when reading ffmpeg video frames.

We had forgotten to unref packets after reading them.
This lead to a memory leak inside of ffmpeg.

source/blender/imbuf/intern/anim_movie.c

@@ -874,6 +874,8 @@ static int ffmpeg_read_video_frame(struct anim *anim, AVPacket *packet)
     if (packet->stream_index == anim->videoStream) {
       break;
     }
+    av_packet_unref(packet);
+    packet->stream_index = -1;
   }
 
   return ret;