[14u] RFR 8237632: Shenandoah: accept NULL fwdptr to cooperate with JVMTI and JFR

[Zhengyu Gu]

Hi,

I would like to backport this patch to 14u.

Shenandoah and JVMTI/JFR leakprofiler all use object mark word's 
'marked' pattern, but for different purpose. Without this patch, it can 
result fatal crash for Shenandoah, as it interpreters JVMTI/JFR visited 
objects as forwarding objects.

The original patch does not apply cleanly, mainly due to line shifts and 
missing JDK-8236888 in JDK14u.

Original bug: https://bugs.openjdk.java.net/browse/JDK-8237632
Original code review: 
https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2020-February/028693.html

14u Webrev: http://cr.openjdk.java.net/~zgu/JDK-8237632/14u/webrev.00/


Test:
   hotspot_gc_shenandoah
   vmTestbase_nsk_jvmti
   vmTestbase_nsk_jdi

Thanks,

-Zhengyu