Incorrect check for calling class in FXMLLoader::getDefaultClassLoader

To reproduce this, run the attached test program as follows:

$ java @build/run.args -Djava.security.manager -Djava.security.policy=build/run.java.policy GetFXMLClassLoader

Testing FXMLLoader::getDefaultClassLoader
***** ERROR: did not get expected exception

On JDK 10 and earlier, this will work and correctly get a security exception. With openjfx11, this fails to get the expected exception.