call_user_func() error with PHP 5.6.38

It seems PHP version 5.6.38 (and may be others) does not handle calls to non-user functions (in our case: class_exists) via call_user_func(). See https://www.drupal.org/project/openlayers/issues/3013374 🐛 openlayers.Types.internal.Error does not exist Fixed for a listing of resulting errors.

To avoid this problem, the attached patch defines wrapping user functions for class_exists(). This works for me.

Thanks for considering it.