[AD] ex_winfull crash on exit [OS X]

[Evert Glebbeek <eglebbk@xxxxxxxxxx>]

I just found ex_winfull crashes on OS X when the windows are destroyed  
at the end.
The cause is the backup and restoration of the "current" display at  
the end of destroy_display(). The sequence of events is:

1. The current display is the windowed display (win).
2. The windowed display (current display) is destroyed.
3. The fullscreen display is destroyed.

When the display is destroyed, the display is made current (to make  
the OpenGL context current), however, when the fullscreen display is  
destroyed, the (now non-existent) windowed display is set as current.  
This means that when it tries to restore the "current" display at the  
end of the function al_set_current_display() crashes. Setting the  
fullscreen window as the "current" display prior to calling  
destroy_display() fixes the crash.

Now, my question is: what should al_get_current_display() return if  
the previous current display was destroyed? NULL?

Evert