locking/rtmutex: Skip remove_waiter() when waiter is not enqueued

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UBUNTU-CVE-2016-1521

The directrun function in directmachine.cpp in Libgraphite in Graphite 2 1.2.4, as used in Mozilla Firefox before 43.0 and Firefox ESR 38.x before 38.6.1, does not validate a certain skip operation, which allows remote attackers to execute arbitrary code, obtain sensitive information, or cause a...