darwin: Eliminate a pthread mutex leak
darwin: Eliminate a pthread mutex leak

Signed-off-by: Jeremy Huddleston <db3cec0cba09acb9cf8fd8bd399a3b27498cf883@apple.com>
Tested-by: Charles Davis <a49bd135bfb1366b165f7022505c53d45b644f43@mines.edu>

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