RFR: 8215395: Allow null oops in Dictionary and JNIHandle verification

[Per Liden]

JDK-8214784 adjusted the Dictionary and JNIHandle verification, but 
failed to take into account that these oops can be null. We should be 
using oopDesc::is_oop_or_null() here rather than just oopDesc::is_oop().

Bug: https://bugs.openjdk.java.net/browse/JDK-8215395
Webrev: http://cr.openjdk.java.net/~pliden/8215395/webrev.0

/Per