RFR: 8025922 JNI access to Strings need to check if the value field is non-null

[Staffan Larsen]

If the String constructor has not completed, then we can't access the String value. It is still possible to do this from JNI, but it should not crash the VM.

This was found while fixing a debugger issue (JDK-8021897) where stepping into the String constructor crashes the JVM. The fix for JDK-8021897 requires changes on the JDK side. Those changes will also include a JDI test for that issue, but won't be pushed until this change reaches jdk8-tl.

bug: https://bugs.openjdk.java.net/browse/JDK-8025922
webrev: http://cr.openjdk.java.net/~sla/8025922/webrev.00/

Thanks,
/Staffan