Apply InnoDB snapshot innodb-5.1-ss2858, part 3.

branches/5.1:

Silence a compilation warning in UNIV_DEBUG.

Approved by:    Marko (via IM)

storage/innobase/include/sync0sync.ic

@@ -197,7 +197,7 @@ mutex_exit(
 {
 	ut_ad(mutex_own(mutex));
 
-	ut_d(mutex->thread_id = ULINT_UNDEFINED);
+	ut_d(mutex->thread_id = (os_thread_id_t) ULINT_UNDEFINED);
 
 #ifdef UNIV_SYNC_DEBUG
 	sync_thread_reset_level(mutex);