RFR (S) 8231197: Shenandoah: JVMTI heap walking cleanup crashes with NULL forwardee

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8231197

Fix:
  https://cr.openjdk.java.net/~shade/8231197/webrev.02/

This reliably reproduces with vmTestbase_nsk_jvmti. The reason is described in the comment there:
weird clash between JVMTI-own walk of heap and Shenandoah. ShenandoahCodeRoots assert also needs
relaxing, and I relaxed it only for JVMTI heap walk.

Testing: hotspot_gc_shenandoah, vmTestbase_nsk_jvmti

-- 
Thanks,
-Aleksey