Allow widget callback to be NULL (STR #2835).

git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@11811 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

CHANGES

@@ -99,6 +99,7 @@ CHANGES IN FLTK 1.3.4					RELEASED: ??? ?? 2016
 
 	Bug fixes
 
+	- Allow widget callback to be NULL, i.e. no callback (STR #2835).
 	- Fixed Fl_Help_View buffer overflow (STR #3275).
 	- Fl_Browser now correctly adjusts its scrollbars when the default
 	  text size is changed with textsize(int) (STR #3057).

src/Fl_Widget.cxx

@@ -343,6 +343,7 @@ Fl_Widget::copy_label(const char *a) {
 */
 void
 Fl_Widget::do_callback(Fl_Widget* o,void* arg) {
+  if (!callback_) return;
   Fl_Widget_Tracker wp(this);
   callback_(o,arg);
   if (wp.deleted()) return;