memory : copy Hadamard matrix to k_rot tensor only if it has buffer assigned to prevent crashes during context shift of unquantized K cache (#27967)

Co-authored-by: Stanisław Szymczyk <sszymczy@gmail.com>
Co-authored-by: AesSedai <7980540+AesSedai@users.noreply.github.com>

src/llama-kv-cache.cpp

@@ -2034,7 +2034,7 @@ void llm_graph_input_k_shift::set_input(const llama_ubatch * ubatch) {
         kv_self->set_input_k_shift(k_shift);
     }
 
-    if (k_rot) {
+    if (k_rot && k_rot->buffer) {
         kv_self->set_input_k_rot(k_rot);
     }
 }