sd-bus: fix exiting event loop when sd_bus_set_exit_on_disconnect is used

sd-bus: fix exiting event loop when sd_bus_set_exit_on_disconnect is used

If sd_bus_set_exit_on_disconnect is used and the bus is part of an event
loop, and the D-Bus connection goes away (e.g.: soft-reboot), sd-bus
will always exit() the program instead of returning from the loop, as
the reference to the event is removed before it is checked.