fix a crash when the size of a label is empty and '\t' is contained

source/gui/detail/window_layout.cpp

@@ -24,6 +24,9 @@ namespace nana
 		//class window_layout
 			void window_layout::paint(core_window_t* wd, bool is_redraw, bool is_child_refreshed)
 			{
+				if (wd->drawer.graphics.empty())
+					return;
+
 				if (nullptr == wd->effect.bground)
 				{
 					if (is_redraw)