RFR: Verifier should check klass pointers before attempting to reach for object size

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/verifier-klass-checks/webrev.01/

Debug a new feature -> corrupt the heap -> crash the Verifier. Verifier should be more resilient
when Klass pointers are NULL, in which case oopDesc::size() is guaranteed to fail.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey