vp8: prevent read from uninitialized memory in decode_mvs

Signed-off-by: Janne Grunau <janne-libav@jannau.net>
(cherry picked from commit 0f0b5d6434)

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

libavcodec/vp8.c

@@ -492,6 +492,7 @@ void decode_mvs(VP8Context *s, VP8Macroblock *mb, int mb_x, int mb_y)
 
     AV_ZERO32(&near_mv[0]);
     AV_ZERO32(&near_mv[1]);
+    AV_ZERO32(&near_mv[2]);
 
     /* Process MB on top, left and top-left */
     #define MV_EDGE_CHECK(n)\