Fix link error of PROCESS_STATE_NONEXISTENT.

* A reference to PROCESS_STATE_NONEXISTENT will cause link error with
  clang or standard C++ compiler because there is no out-of-class definition,
  although g++ is allowing it.
  Use +PROCESS_STATE_NONEXISTENT to get the value instead of reference.

Change-Id: I62049584b94e6847b64b572f6ef6c3e9dcc573eb