lib/avahi/Client: relax assertion after COLLISION/REGISTERING

Turns out that `AVAHI_CLIENT_S_COLLISION` can occur after
`AVAHI_CLIENT_S_RUNNING`, and `connected==true`.  Relaxing this fixes
a bogus assertion failure.

src/lib/avahi/Client.cxx

@@ -79,7 +79,7 @@ Client::ClientCallback(AvahiClient *c, AvahiClientState state) noexcept
 
 	case AVAHI_CLIENT_S_COLLISION:
 	case AVAHI_CLIENT_S_REGISTERING:
-		assert(!connected);
+		connected = false;
 
 		for (auto *l : listeners)
 			l->OnAvahiChanged();