Fix invalid channel selections leading to event sub disconnect

Repeated invalid POSTs to /helix/eventsub/subscriptions will lead to a
disconnect of the event sub websocket connection.
Thus one single invalid channel selection could tear down any existing
subscriptions with it.

plugins/twitch/channel-selection.cpp

@@ -197,7 +197,7 @@ void TwitchChannel::ResolveVariables()
 	_name.ResolveVariables();
 }
 
-bool TwitchChannel::IsValid(const std::string &id) const
+bool TwitchChannel::IsValid(const std::string &id)
 {
 	return id != "invalid" && !id.empty();
 }