NUVIE: Fix Ultima 6 crash on loading save

[PushmePullyu]

Fix a use-after-free bug. The chain of events causing it is as follows:

In-game the user changes the current view (e.g. by pressing TAB to go to inventory view). The user next presses ESC to open the GameMenuDialog and then clicks the "Load Game" button. A save slot is selected and then loaded.

The resulting call chain is:

GUI_Button::Activate_button()
  GameMenuDialog::callback()
    GameMenuDialog::close_dialog() <-- This marks GameMenuDialog
                                       for deletion by the GUI
    g_engine->loadGameDialog()
      GUI::AddWidget() <-------------- This deletes GameMenuDialog
                                       and its children, including
                                       the "Load Game" button

Control then returns to GUI_Button::Activate_button() with "this" containing the address of the freed button.

An attempt to call Redraw() finally dereferences the invalid pointer.

This is fixed by calling loadGameDialog() before close_dialog().

[dreammaster]

Nice. Thanks for tracking down and fixing the problem.