RFR(T) 8229318: Shenandoah: Assertion failure in nightly test

[Zhengyu Gu]

Please review this trivial patch that fixes a bad assertion. The same 
assertions are coded correctly elsewhere.

Bug: https://bugs.openjdk.java.net/browse/JDK-8229318
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8229318/webrev.00/

Test:
   hotspot_gc_shenandoah fastdebug.

Thanks,

-Zhengyu