RFR: 8240529: CheckUnhandledOops breaks NULL check in Modules::define_module

[Stefan Karlsson]

Hi all,

Please review this small fix to CheckUnhandledOops.

https://cr.openjdk.java.net/~stefank/8240529/webrev.01
https://bugs.openjdk.java.net/browse/JDK-8240529

'loader' is an unhandled oop that is alive across a safepoint check and 
then is used for a NULL check. When the safepoint check executes the 
value of the oop is changed to 0xff...ff1. If the oop was NULL then the 
check now starts to fail. When this happens 
ClassLoader::_exploded_entries isn't populated as expected, causing 
class loading failures with exploded builds.

Thanks,
StefanK