fix a race condition deep in mutex API.
fix a race condition deep in mutex API.

after performing a wakeup(), be sure not to access any fields inside the
mutex as it could have gone away.

pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/pfl,pscedu/pfl,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/pfl,pscedu/pfl