tools : fix uninitialized llama_batch in server (#13436)

* add constructor to initialize server_context::batch, preventing destructor's call to llama_batch_free from causing an invalid free()

* Update tools/server/server.cpp

Co-authored-by: Xuan-Son Nguyen <thichthat@gmail.com>

* use C++11 initializer syntax

* switch from Copy-list-initialization to Direct-list-initialization

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Co-authored-by: Xuan-Son Nguyen <thichthat@gmail.com>

tools/server/server.cpp

@@ -1862,7 +1862,7 @@ struct server_context {
 
     llama_context_params cparams_dft;
 
-    llama_batch batch;
+    llama_batch batch {};
 
     bool clean_kv_cache = true;
     bool add_bos_token  = true;