RFR: 8021591 : (s) Additional explicit null checks

[Mike Duigou]

Hello all;

This patch adds some missing checks for null that, according to interface contract, should be throwing NPE. It also improves the existing tests to check for these cases. 

http://cr.openjdk.java.net/~mduigou/JDK-8021591/0/webrev/

The changes to src/share/classes/java/util/concurrent/ConcurrentHashMap.java will be synchronized separately with the jsr166 workspace. They are part of this review to avoid test failures.

Mike