fixed crash when CustomMenu doesn't point to a real file

ChangeLog

@@ -1,5 +1,8 @@
  (Format: Year/Month/Day)
 Changes for 1.0.0:
+*07/04/19:
+   * Fixed crash when CustomMenu command doesn't point to a file (Mark)
+     FbCommands.cc
 *07/04/17:
    * cosmetic changes (Mathias, thanks Slava Semushin)
      configure.in Screen.cc Keys.cc main.cc FbTk/MenuItem.hh FbTk/Timer.hh

src/FbCommands.cc

@@ -281,6 +281,8 @@ void ShowCustomMenuCmd::execute() {
         return;
     m_menu = MenuCreator::createFromFile(custom_menu_file,
             screen->screenNumber(), true);
+    if (!m_menu.get())
+        return;
     ::showMenu(*screen, **m_menu);
 }