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)

source/paint/detail/native_paint_interface.cpp

@@ -156,6 +156,9 @@ namespace detail
 
 	nana::size text_extent_size(drawable_type dw, const nana::char_t * text, std::size_t len)
 	{
+		if (nullptr == dw || nullptr == text || 0 == len)
+			return{};
+
 		nana::size extents = raw_text_extent_size(dw, text, len);
 
 		const nana::char_t* const end = text + len;