Initialize all of class Thread's fields.

Change-Id: I28f1539bbcfebe06e62de692d306b479ed3eaead
diff --git a/src/monitor.cc b/src/monitor.cc
index cc74838..0ef5fb4 100644
--- a/src/monitor.cc
+++ b/src/monitor.cc
@@ -126,7 +126,7 @@
 void Monitor::AppendToWaitSet(Thread* thread) {
   DCHECK(owner_ == Thread::Current());
   DCHECK(thread != NULL);
-  DCHECK(thread->wait_next_ == NULL);
+  DCHECK(thread->wait_next_ == NULL) << thread->wait_next_;
   if (wait_set_ == NULL) {
     wait_set_ = thread;
     return;