wgl: Initialize DEVMODE struct

Otherwise the dmDriverExtra field might be uninitialized and have a nonzero
value, which can cause the API implementation to smash the stack when copying
to the output struct.

Cc: mesa-stable
Reviewed-by: Joshua Ashton <joshua@froggi.es>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27998>

src/gallium/frontends/wgl/stw_device.c

@@ -74,7 +74,7 @@ static int
 get_refresh_rate(void)
 {
 #ifndef _GAMING_XBOX
-   DEVMODE devModes;
+   DEVMODE devModes = { .dmSize = sizeof(DEVMODE) };
 
    if (EnumDisplaySettings(NULL, ENUM_CURRENT_SETTINGS, &devModes)) {
       /* clamp the value, just in case we get garbage */