git: openjdk/loom: fibers: Fix issue with calling GetCurrentContendedMonitor on an unmounted vthread. The monitor_ptr arg was not being set to null. Fix existing test to detect this issue.

[duke]

Changeset: ff234bf4
Author:    Chris Plummer <chris.plummer at oracle.com>
Date:      2020-11-19 20:51:00 +0000
URL:       https://github.com/openjdk/loom/commit/ff234bf4

Fix issue with calling GetCurrentContendedMonitor on an unmounted vthread. The monitor_ptr arg was not being set to null. Fix existing test to detect this issue.

! src/hotspot/share/prims/jvmtiEnv.cpp
! test/hotspot/jtreg/serviceability/jvmti/vthread/VThreadMonitorTest/libVThreadMonitorTest.c