server : properly handle null llama_context (#25868)

Co-authored-by: Stanisław Szymczyk <sszymczy@gmail.com>

tools/server/server-context.cpp

@@ -1152,6 +1152,11 @@ private:
             return false;
         }
 
+        if (ctx_tgt == nullptr) {
+            SRV_ERR("failed to create_context with model '%s'\n", params_base.model.path.c_str());
+            return false;
+        }
+
         vocab = llama_model_get_vocab(model_tgt);
 
         n_ctx = llama_n_ctx(ctx_tgt);