First cast to LONG, then to DWORD.

Reported by MinGW GCC 10 with -Woverflow.
Signed-off-by: Rémi Bernon <rbernon@codeweavers.com>
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>