dbus-socket: fix check of Listen* arguments

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)
(cherry picked from commit 78fb13b38572f6649f1e9822ef6acb8ca4952c12)