RFR: 6640188: Methods com.sun.tools.attach.VirtualMachine.load... don't throw NullPointerException

[gary.adams at oracle.com]

Here's a simple fix to add explicit checks for the loadAgentXXX methods to
ensure NPE is thrown when null arguments are passed. Several other 
management
methods have similar provider not null checks, they just were not 
present for
agent path and library methods. This should remove the spec ambiguity with
AgentLoadException which was being thrown.

   Issue: https://bugs.openjdk.java.net/browse/JDK-6640188
   Webrev: http://cr.openjdk.java.net/~gadams/6640188/