[GPU] fix unit-test seg fault error on dGPU (#16879)

Signed-off-by: Min, Byungil <byungil.min@intel.com>

src/plugins/intel_gpu/src/graph/layout_optimizer.cpp

@@ -760,7 +760,7 @@ static bool is_node_for_onednn(reduce_node const& node, format preferred_format)
     auto reduce_prim = node.get_primitive();
 
     if (input.get_output_layout().data_type == data_types::f32
-        && node.get_users().front()->get_output_layout().data_type == data_types::f32) {
+        && node.get_output_layout().data_type == data_types::f32) {
         return false;
     }