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.

test/TestMain.m

@@ -116,13 +116,4 @@ OF_APPLICATION_DELEGATE(TestMain)
 		}
 	}
 }
-
-- (void)applicationWillTerminate
-{
-	/* Make sure they don't get deallocated after al_uninstall_system() */
-	display = nil;
-	eventQueue = nil;
-
-	al_uninstall_system();
-}
 @end