RFR (S) 8218851: JVM crash in custom classloader stress test, JDK 12 & 13

[coleen.phillimore at oracle.com]

Summary: Handle NULL and unloaded constraint class in loader constraint 
table, also cope with unloaded but not cleaned out klass in loader 
constraint entries.

Tested overnight with the application that caused the crash.  Tested 
with hs tier1-6.   Tested JCKs with ZGC.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8218851.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8218851

Thanks,
Coleen