Adapt gix-sec to changes in windows-sys

Bumping the `windows-sys` dependency of `gix-sec` from 0.52.0 to
0.59.9 broke a Windows API function call that passed a literal `0`
for a null handle, because the interface became a bit more strongly
typed. This changes the argument to `std::ptr::null_mut()`.

Author: EliahKagan

Diff for: gix-sec/src/identity.rs

@@ -200,7 +200,7 @@ mod impl_ {
             }
 
             let mut is_member = 0;
-            if CheckTokenMembership(0, token_owner, &mut is_member) == 0 {
+            if CheckTokenMembership(std::ptr::null_mut(), token_owner, &mut is_member) == 0 {
                 error!("Couldn't check if user is an administrator");
             }