GDScript LSP: Fix wrong error checks added in #39385

Reverts `latest_client_id` back to 0, as I misunderstood how the client
IDs are assigned and, without further testing and debugging, I can't
say if this was a bug or a valid default value.
Similarly, a `latest_client_id` of -1 is no longer raising an error.

Fixes #39548.

(cherry picked from commit 786f4ada35)

modules/gdscript/language_server/gdscript_language_protocol.h

@@ -70,7 +70,7 @@ private:
 
 	HashMap<int, Ref<LSPeer> > clients;
 	Ref<TCP_Server> server;
-	int latest_client_id = -1;
+	int latest_client_id = 0;
 	int next_client_id = 0;
 
 	Ref<GDScriptTextDocument> text_document;