Static analysis fix: dereference of a NULL pointer.

src/video/cocoa/SDL_cocoawindow.m

@@ -1464,9 +1464,7 @@ Cocoa_SetWindowGrab(_THIS, SDL_Window * window, SDL_bool grabbed)
         CGDisplayMoveCursorToPoint(kCGDirectMainDisplay, cgpoint);
     }
 
-    if ( window->flags & SDL_WINDOW_FULLSCREEN ) {
-        SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
-
+    if ( data && (window->flags & SDL_WINDOW_FULLSCREEN) ) {
         if (SDL_ShouldAllowTopmost() && (window->flags & SDL_WINDOW_INPUT_FOCUS)) {
             /* OpenGL is rendering to the window, so make it visible! */
             [data->nswindow setLevel:CGShieldingWindowLevel()];