RFR (trivial) 8189794: Assert in InstanceKlass::cast called from Exceptions::new_exceptions

[coleen.phillimore at oracle.com]

Summary: Fix call to InstanceKlass::cast to only be after verifying 
class is non-null.

Also fixed an unnecessary InstanceKlass::cast found while looking for 
similar instances of this bug.  I didn't find this same bug anywhere 
else.  Tested with internal vm.gc.locker tests, and with JPRT which is 
equivalent to tier1 only faster at the moment.

open webrev at http://cr.openjdk.java.net/~coleenp/8189794.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8189794

Thanks,
Coleen