[server] Cleanup a memory leak on exit

There are a couple memory leaks on exit of the server. This hides others.
After cleaning this up, you can see leaks on slots. But that is another
patch to be sent after this.

examples/server/server.cpp

@@ -673,6 +673,8 @@ struct server_context {
             llama_free_model(model);
             model = nullptr;
         }
+
+	llama_batch_free(batch);
     }
 
     bool load_model(const gpt_params & params_) {