RFR(XS) 8191852: Null pointer dereference in ciKlass::get_Klass of ciKlass.hpp:58

[dean.long at oracle.com]

https://bugs.openjdk.java.net/browse/JDK-8191852
http://cr.openjdk.java.net/~dlong/8191852/webrev/

Our static analysis tool was complaining about a possible null pointer 
dereference in ciKlass::get_Klass(), because of this code:

237.      _holder = CURRENT_ENV->get_instance_klass(fd->field_holder());
[...]
240.      Klass* k = _holder->get_Klass();

so I added NULL checks in get_instance_klass and a few other similar 
functions.

dl