RFR(XS): JDK-8043225: Make whitebox API functions more stable

[Kirill Zhaldybin]

Hi!

Could you please review the following patch for WB methods?
To make API functions more stable I added checks on null values where 
applicable.

CR:
https://bugs.openjdk.java.net/browse/JDK-8043225

Webrev:
top: http://cr.openjdk.java.net/~ppunegov/kzhaldybin/8043225/webrev/
113 lines changed: 92 ins; 0 del; 21 mod;
hotspot: 
http://cr.openjdk.java.net/~ppunegov/kzhaldybin/8043225/hotspot/webrev/
20 lines changed: 0 ins; 0 del; 20 mod;
Tests:
jprt

Thanks,
Kirill