mutex::lock must not throw

[viboes]

The Mutex requirements say

m.unlock();

Requires:

    The current thread owns m. 
Synchronization:

    This operation synchronizes with subsequent lock operations that obtain ownership on the same object. 
Effects:

    Releases a lock on m by the current thread. 
Return type:

    void. 
Throws:

    Nothing. 

However mutex::unlock throws an exception.