uucore/uptime: remove unreachable code on Windows

[cakebaker]

On lines 337 - 339 we have the following code:

if result == 0 || buffer.is_null() {
    continue;
}

On line 341 we check whether the buffer is not null. Due to the code above buffer can't be null at this point, thus the check is unnecessary and the else part is unreachable.

let username = if !buffer.is_null() {
    // do something
} else {
    // do something else
};