Check for NULL joystick in SDL_JoystickGetGUID()

src/joystick/SDL_joystick.c

@@ -763,6 +763,11 @@ SDL_JoystickGUID SDL_JoystickGetDeviceGUID(int device_index)
 /* return the guid for this opened device */
 SDL_JoystickGUID SDL_JoystickGetGUID(SDL_Joystick * joystick)
 {
+    if (!SDL_PrivateJoystickValid(joystick)) {
+        SDL_JoystickGUID emptyGUID;
+        SDL_zero( emptyGUID );
+        return emptyGUID;
+    }
     return SDL_SYS_JoystickGetGUID( joystick );
 }