xtt: Fix subwindow segfault when updating context qt

xtt/lib/glow/qt/glow_draw_qt.cqt

@@ -2008,9 +2008,12 @@ void GlowDrawQt::pop_customcolors()
 {
   if (customcolors_cnt <= 0) {
     printf("** Customcolor stack disorder\n");
+    return;
   }
-  for (int i = 0; i < customcolors_cnt - 1; i++)
+  for (int i = 0; i < customcolors_cnt - 1; i++) {
     customcolors[i] = customcolors[i + 1];
+    customcolors[i + 1] = NULL;
+  }
   customcolors_cnt--;
 }