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>
