Fix engine crash in SV_SetClient_Ping by ensuring gentity_t::client is never null

- In AllocateClientArray, link g_entities[i+1].client to game.clients[i] immediately.
- In FreeClientArray, link g_entities[i+1].client to a static dummy client instead of nullptr.
- This prevents race conditions during map restarts/shutdowns where the engine might access client structures after they are freed but before the entity array is reset.

Author: google-labs-jules[bot]

src/server/gameplay/g_clients.cpp

@@ -40,6 +40,8 @@ namespace {
 		if (gi.TagFree)
 			gi.TagFree(ptr);
 	}
+
+	gclient_t dummy_client;
 }
 
 void AllocateClientArray(int maxClients) {
@@ -78,7 +80,7 @@ void FreeClientArray() {
 	// [KEX]: Unlink client pointers
 	if (g_entities && game.clients) {
 		for (int i = 0; i < game.maxClients; i++) {
-			g_entities[i + 1].client = nullptr;
+			g_entities[i + 1].client = &dummy_client;
 		}
 	}