RFR(XS): 8217348: assert(thread->is_Java_thread()) failed: just checking

[Yangfei (Felix)]

Hi,

Please review this backport of one trivial change fixing an obvious typo:

Original Bug:
    https://bugs.openjdk.java.net/browse/JDK-8217348 
    http://hg.openjdk.java.net/jdk/jdk13/rev/09ee0bd26bda 

This was once biting us in our internal development. 
Original patch does not apply cleanly to 8u due to changes in surrounding context. 

8u webrev:
    http://cr.openjdk.java.net/~fyang/8217348-8u/webrev.00 

Testing:
    Performed full jtreg tests with a fastdebug build.

Thanks,
Felix