lock: Don't call pthread_mutex_destroy() on a global mutex

[teor2345]

Closes bug 31736.

[coveralls]

Pull Request Test Coverage Report for Build 6394

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 60.307%

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Totals
Change from base Build 6284:	0.001%
Covered Lines:	42887
Relevant Lines:	71114

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💛 - Coveralls