fix a textbox line-wrap issue that only clears first line

include/nana/gui/widgets/skeletons/textbase.hpp

@@ -407,6 +407,8 @@ namespace skeletons
 		{
 			std::deque<string_type>().swap(text_cont_);
 			attr_max_.reset();
+			text_cont_.emplace_back();	//text_cont_ must not be empty
+
 			_m_saved(std::string());
 		}
 

source/gui/widgets/skeletons/text_editor.cpp

@@ -1495,6 +1495,7 @@ namespace nana{	namespace widgets
 
 			_m_reset();
 			behavior_->pre_calc_lines(width_pixels());
+
 			render(API::is_focus_ready(window_));
 			_m_scrollbar();
 			return true;
@@ -1747,6 +1748,7 @@ namespace nana{	namespace widgets
 		{
 			textbase_.erase_all();
 			_m_reset();
+			behavior_->pre_calc_lines(width_pixels());
 			put(std::move(str));
 		}