Fix use-after-free when pumping the event loop after SDL_DestroyWindow()

Closing the window is asynchronous, but we free the window data immediately,
so we can get an updateLayer callback before the window is really destroyed which
will cause us to access the freed memory.

Clearing the content view will cause it to be immediately released, so no further
updateLayer callbacks will occur.

src/video/cocoa/SDL_cocoawindow.m

@@ -1816,6 +1816,8 @@ Cocoa_DestroyWindow(_THIS, SDL_Window * window)
         [data->listener close];
         [data->listener release];
         if (data->created) {
+            /* Release the content view to avoid further updateLayer callbacks */
+            [data->nswindow setContentView:nil];
             [data->nswindow close];
         }