libavcodec/vp3.c:816: error: Uninitialized Value
  The value read from motion_x[_] was never initialized.

libavcodec/vp3.c:816:39: 
814.                 if (s->chroma_y_shift) {
815.                     if (s->macroblock_coding[current_macroblock] == MODE_INTER_FOURMV) {
816.                         motion_x[0] = RSHIFT(motion_x[0] + motion_x[1] +
                                           ^
817.                                              motion_x[2] + motion_x[3], 2);
818.                         motion_y[0] = RSHIFT(motion_y[0] + motion_y[1] +
