Fix stale current_ pointer when deleting Fl_Group

If a user program accidentally deletes the "current" group, then
the pointer would still point at the deleted widget. This commit
prevents this and makes the Fl_Group's parent the current group.

Fixes issue #88.

src/Fl_Group.cxx

@@ -452,6 +452,8 @@ void Fl_Group::clear() {
   widgets' destructors would be called twice!
 */
 Fl_Group::~Fl_Group() {
+  if (current_ == this)
+    end();
   clear();
 }