The defer functions were checking the local variable, and would therefore
not be executed, as the function returned if an error occurred.

Perhaps best illustrated when renaming the local variables;

    if fifos.Stdin != "" {
        l, err1 := winio.ListenPipe(fifos.Stdin, nil)
        if err1 != nil {
            return nil, errors.Wrapf(err1, "failed to create stdin pipe %s", fifos.Stdin)
        }
        defer func(l net.Listener) {
            if err1 != nil {
                l.Close()
            }
        }(l)
        // ...
    }

    if fifos.Stdout != "" {
        l, err2 := winio.ListenPipe(fifos.Stdout, nil)
        if err2 != nil {
            return nil, errors.Wrapf(err2, "failed to create stdout pipe %s", fifos.Stdout)
        }
        defer func(l net.Listener) {
            if err2 != nil {
                l.Close()
            }
        }(l)
        // ....
    }

This patch changes the function to use a named return variable, and to use
a single `defer()` that closes all pipes.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>