Add missing asignment of return code in wait_for_thread_etc().

While it was detected that the thread is in the destruction phase
and that it was necessary to wait and then have a valid status code
in the death entry, that status code wasn't actually returned. This
lead to uninitialized values for the return code even though
wait_for_thread[_etc]() would return B_OK.

src/system/kernel/thread.cpp

@@ -2563,6 +2563,9 @@ wait_for_thread_etc(thread_id id, uint32 flags, bigtime_t timeout,
 			// middle of the cleanup.
 			acquire_sem(exitSem);
 			status = B_OK;
+
+			if (_returnCode != NULL)
+				*_returnCode = death.status;
 		}
 	}