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.

(cherry picked from commit 93a59b1a)
(cherry picked from commit 1e4257da)