sd-bus: break the loop in bus_ensure_running() if the bus is not connecting

This might fix #17025:
> the call trace is
> bus_ensure_running -> sd_bus_process -> bus_process_internal -> process_closeing --> sd_bus_close
>                                                                                  |
>                                                                                  \-> process_match

We ended doing callouts to the Disconnected matches from bus_ensure_running()
and shouldn't. bus_ensure_running() should never do callouts. This change
should fix this however: once we notice that the connection is going down we
will now fail instantly with ENOTOCONN instead of calling any callbacks.