libsystemd: fix compiler warning in property_get_set_callbacks_run()

libsystemd: fix compiler warning in property_get_set_callbacks_run()

gcc (4.8.2, arm) doesn't understand that vtable_property_get_userdata()
will always set 'u' when it returns > 0. Hence, the warning is bogus,
but anyway.

  src/libsystemd/sd-bus/bus-objects.c:510:19: warning: 'u' may be used uninitialized in this function [-Wmaybe-uninitialized]

(and yes, indeed, even the reported line numbers are bogus in this case)