Return if unpacking motion vectors for a keyframe

Originally committed as revision 15502 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vp3.c

@@ -812,6 +812,7 @@ static int unpack_vectors(Vp3DecodeContext *s, GetBitContext *gb)
     int current_fragment;
 
     if (s->keyframe) {
+        return 0;
     } else {
         memset(motion_x, 0, 6 * sizeof(int));
         memset(motion_y, 0, 6 * sizeof(int));