Only call al_*_destroy if Allegro is initialized.

Otherwise, it would crash if al_uinstall_system() has already been
called. Handling it this way eliminates the need to dealloc all objects
before calling al_uninstall_system(), which meant that it was the users
repsonsibility to call al_uninstall_system() after the user made sure
all objects are deallocated. Now the user does not get to see any
al_*() function.