Addresses issue raised by Michael Sartain on fltk.coredev on 02/11/2016;

prevent problems with Fl_Tree::_item_focus remaining valid after clear().
Applied his patch, plus extra checks added to Fl_Tree_Item's dtor.



git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@11173 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

src/Fl_Tree.cxx

@@ -1135,6 +1135,7 @@ void Fl_Tree::clear() {
   if ( ! _root ) return;
   _root->clear_children();
   delete _root; _root = 0;
+  _item_focus = 0;
 } 
 
 /// Clear all the children for \p 'item'.

src/Fl_Tree_Item.cxx

@@ -116,6 +116,11 @@ Fl_Tree_Item::~Fl_Tree_Item() {
   _usericon = 0;		// user handled allocation
 #if FLTK_ABI_VERSION >= 10304
   if ( _userdeicon ) delete _userdeicon;	// delete our copy (if any) for deactivated icon
+#endif
+#if FLTK_ABI_VERSION >= 10303
+  // focus item? set to null
+  if ( _tree && this == _tree->_item_focus )
+    { _tree->_item_focus = 0; }
 #endif
   //_children.clear();		// array's destructor handles itself
 }