dbus-socket: fix check of Listen* arguments
    
    We checked the wrong field, which was always NULL here, so we would always
    reject the assignment. We would also print the wrong string in the error
    message:
    
    $ sudo systemd-run --socket-property ListenFIFO=/tmp/fifo3 cat
    Failed to start transient socket unit: Invalid socket path: FIFO
    
    (cherry picked from commit aeecab3804aae973577f36880af4b7799e4eb7d5)