When debugging using the test case from 4304181 hitting 'step' on the
current thread which has exited periodically results in an assertion
in HotSpot Client VM :-

#
# HotSpot Virtual Machine Error, assertion failure
#
# assert(is_global_handle(handle), "Invalid delete of global JNI handle")
#
# Error ID: D:\workspaces\hotspot\src\share\vm\runtime\jniHandles.hpp, 164
#

On investigation it turns out that the JDWP stub is attemping to delete
a global JNI reference that has not been created.

Note that this bug is different to JDWP bug 4308025 but it is
duplicated in a similiar manner.

I have analyzed the crash - see suggested fix for details.