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 aeecab38)