Add precondition guards to pthread_mutex calls

[Danappelxx]

While in the current configuration lock/unlock shouldn't ever fail, it seems polite to guard against basic misuse in debug builds.

[kperryua]

I think you may want to pull the result of the lock/unlock calls out of the precondition(). According to the docs:

In -Ounchecked builds, condition is not evaluated, but the optimizer may assume that it always evaluates to true. Failure to satisfy that assumption is a serious programming error.

So in -Ounchecked builds, there will be no locking at all in these paths.

[phausler]

err good call @kperryua; will adjust that