avcodec/vp9mvs: fix misaligned access when clearing VP9mv

Fixes runtime error: member access within misaligned address
<addr> for type 'av_alias64', which requires 8 byte alignment.

VP9mv is aligned to 4 bytes, so instead doing 8 bytes clear, let's do
2 times 4 bytes.

Signed-off-by: Kacper Michajłow <kasper93@gmail.com>
Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vp9mvs.c

@@ -294,7 +294,8 @@ void ff_vp9_fill_mv(VP9TileData *td, VP9mv *mv, int mode, int sb)
     VP9Block *b = td->b;
 
     if (mode == ZEROMV) {
-        AV_ZERO64(mv);
+        AV_ZERO32(&mv[0]);
+        AV_ZERO32(&mv[1]);
     } else {
         int hp;