fix crash when trying to set the title on a window that's already closed

ChangeLog

@@ -1,4 +1,9 @@
  (Format: Year/Month/Day)
+Changes for 1.1.2
+*08/09/18:
+   * Fix crash when using SetTitleDialog on a window that's been closed (Mark)
+     CurrentWindowCmd.cc
+-----------------------------
 Changes for 1.1.1
 *08/09/14:
    * Fixed a minor pixmap resource leak (Henrik)

src/CurrentWindowCmd.cc

@@ -467,13 +467,17 @@ void SetLayerCmd::real_execute() {
 }
 
 namespace {
-class SetTitleDialog: public TextDialog {
+class SetTitleDialog: public TextDialog, public FbTk::Observer {
 public:
     SetTitleDialog(FluxboxWindow &win, const string &title):
         TextDialog(win.screen(), title), window(win) {
+        win.dieSig().attach(this);
         setText(win.title());
     }
 
+    // only attached signal is window destruction
+    void update(FbTk::Subject *subj) { delete this; }
+
 private:
     void exec(const std::string &text) {
         window.winClient().setTitle(text);