RFR (XXS) JDK-8046662: Check JNI ReleaseStringChars / ReleaseStringUTFChars verify_guards test inverted

[David Simms]

Hello,

Looking to get a *quick* review for small fix (regression introduced 
yesterday messing with nightly testing):

Bug URL: https://bugs.openjdk.java.net/browse/JDK-8046662
Web Review: http://cr.openjdk.java.net/~dsimms/8046662/

-Xcheck:jni broken without this fix.

Testing: re-ran internal tests which failed last night.

Cheers
/David Simms