socket: avoid memory leak on incomplete SocketPort object

socket: avoid memory leak on incomplete SocketPort object

==1==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 17 byte(s) in 1 object(s) allocated from:
    #0 0x7fc096c7243b in strdup (/lib64/libasan.so.8+0x7243b)
    #1 0x7fc095db3899 in bus_socket_set_transient_property ../src/core/dbus-socket.c:386
    #2 0x7fc095db5140 in bus_socket_set_property ../src/core/dbus-socket.c:460
    #3 0x7fc095dd20f1 in bus_unit_set_properties ../src/core/dbus-unit.c:2473
    #4 0x7fc095d87d53 in transient_unit_from_message ../src/core/dbus-manager.c:1025
    #5 0x7fc095d8872f in method_start_transient_unit ../src/core/dbus-manager.c:1112
    #6 0x7fc0944ddf4f in method_callbacks_run ../src/libsystemd/sd-bus/bus-objects.c:406
    #7 0x7fc0944e7854 in object_find_and_run ../src/libsystemd/sd-bus/bus-objects.c:1319
    #8 0x7fc0944e8f03 in bus_process_object ../src/libsystemd/sd-bus/bus-objects.c:1439
    #9 0x7fc09454ad78 in process_message ../src/libsystemd/sd-bus/sd-bus.c:3011
    #10 0x7fc09454b302 in process_running ../src/libsystemd/sd-bus/sd-bus.c:3053
    #11 0x7fc09454e158 in bus_process_internal ../src/libsystemd/sd-bus/sd-bus.c:3273
    #12 0x7fc09454e2f2 in sd_bus_process ../src/libsystemd/sd-bus/sd-bus.c:3300
    #13 0x7fc094551a59 in io_callback ../src/libsystemd/sd-bus/sd-bus.c:3642
    #14 0x7fc094727830 in source_dispatch ../src/libsystemd/sd-event/sd-event.c:4187
    #15 0x7fc094731009 in sd_event_dispatch ../src/libsystemd/sd-event/sd-event.c:4808
    #16 0x7fc094732124 in sd_event_run ../src/libsystemd/sd-event/sd-event.c:4869
    #17 0x7fc095f7af9f in manager_loop ../src/core/manager.c:3242
    #18 0x41cc7c in invoke_main_loop ../src/core/main.c:1937
    #19 0x4252e0 in main ../src/core/main.c:3072
    #20 0x7fc092a4a50f in __libc_start_call_main (/lib64/libc.so.6+0x2750f)

SUMMARY: AddressSanitizer: 17 byte(s) leaked in 1 allocation(s).