The JDWP 1.6 specification at:
  http://java.sun.com/javase/6/docs/platform/jpda/jdwp/jdwp-protocol.html#JDWP_ThreadReference_OwnedMonitorsStackDepthInfo
states as follows:
  ...
  INVALID_THREAD Passed thread is null, is not a valid thread or has exited.
  ...

However, test shows that if passed thread reference is null then the ThreadReference.OwnedMonitorsStackDepthInfo command
returns NOT_SUSPENDED error code instead of INVALID_THREAD.

Note that this command returns correct INVALID_THREAD error code:
  - for non-thread object reference (not a valid thread)
  - for a thread which has exited but was suspended before.