vc1dec.c: Don't use uninitialized values of dmv_[x, y]

MVDATA may or may not be transmitted. If it is not, both
dmv_x and dmv_y is to be assumed zero.

This may not trigger wrong picture in all systems, but
it's a bug nevertheless. Fixes SA10116.vc1 on my 64-bit
Windows 7.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavcodec/vc1dec.c

@@ -3954,6 +3954,7 @@ static int vc1_decode_p_mb_intfr(VC1Context *v)
                 vc1_mc_4mv_chroma4(v);
             } else {
                 mvbp = ff_vc1_mbmode_intfrp[v->fourmvswitch][idx_mbmode][2];
+                dmv_x = dmv_y = 0;
                 if (mvbp) {
                     get_mvdata_interlaced(v, &dmv_x, &dmv_y, 0);
                 }