[vpr] format fix 2

Author: oscarcheng105

vpr/src/draw/search_bar.cpp

@@ -138,10 +138,11 @@ void search_and_highlight(GtkWidget* /*widget*/, ezgl::application* app) {
 bool highlight_rr_nodes(int hit_node) {
     t_draw_state* draw_state = get_draw_state_vars();
 
+    //TODO: fixed sized char array may cause overflow.
     char message[250] = "";
-    auto& device_ctx = g_vpr_ctx.device(); 
 
     if (hit_node != OPEN) {
+        const auto& device_ctx = g_vpr_ctx.device();
         auto nodes = draw_expand_non_configurable_rr_nodes(hit_node);
         for (auto node : nodes) {
             if (draw_state->draw_rr_node[node].color != ezgl::MAGENTA) {