Fixed a potential crash if Fl_Double_Window::flush() was called before show().

Thanks to Mathieu Peyrega for the patch in fltk.general [1] on Nov 14, 2011,
although I implemented it differently.
[1] http://www.fltk.org/newsgroups.php?gfltk.general+v:33946


git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@9188 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

CHANGES

@@ -1,6 +1,8 @@
 
 CHANGES IN FLTK 1.3.1
 
+	- Fixed a potential crash if Fl_Double_Window::flush() was called
+	  before show()
 	- Fixed regression (in FLTK 1.3.0) that could clear a radio
 	  button by using the keyboard (space or shortcut) (STR #2748)
 	- Fixed fl_pie() drawing too small on X11 (STR #2703)

src/Fl_Double_Window.cxx

@@ -326,6 +326,7 @@ void Fl_Double_Window::flush() {flush(0);}
 void Fl_Double_Window::flush(int eraseoverlay) {
   make_current(); // make sure fl_gc is non-zero
   Fl_X *myi = Fl_X::i(this);
+  if (!myi) return; // window not yet created
   if (!myi->other_xid) {
 #if USE_XDBE
     if (can_xdbe()) {