GDBusServer: Avoid handling incoming connections after stop() has been called

GDBusServer: Avoid handling incoming connections after stop() has been called

Without this fix, we'd sometimes run code after stop() and finalize()
to handle incoming requests. This was observed in the gdbus-peer test
case occasionally crashing:

 $ ./gdbus-peer
 /gdbus/peer-to-peer: OK
 /gdbus/delayed-message-processing: OK
 /gdbus/nonce-tcp:
 GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `GDBusServer'
 aborting...

Signed-off-by: David Zeuthen <davidz@redhat.com>