RFR: 8235257: Split 'assert(loader != NULL && oopDesc::is_oop(loader), "loader must be oop")'

[Stefan Karlsson]

Hi all,

Please review this trivial patch to split this assert into two asserts.

https://cr.openjdk.java.net/~stefank/8235257/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8235257

We keep hitting this assert in testing and it's unclear if the loader is 
NULL or if it is a bad oop. I propose that we split it in two.

Thanks,
StefanK