RFR(XXS) 8231275 Remove null check in the beginning of SystemDictionary::load_shared_class()

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8231275
http://cr.openjdk.java.net/~iklam/jdk14/8231275-remove-null-check-in-load-shared-class.v01/

SystemDictionary::load_shared_class() should not be called with a NULL 
class. I replaced the null check with an assert.

Thanks
- Ioi