RFR: 6584008: jvmtiStringPrimitiveCallback should not be invoked when string value is null

[Andrew Haley]

Webrev at http://cr.openjdk.java.net/~aph/JDK-6584008/

I have attached a reporducer to
https://bugs.openjdk.java.net/browse/JDK-6584008 but I'm not sure how
to turn it into a proper JTREG test.

Andrew.