try_mutex locks throw if unlocked twice

[SVN r34249]

include/boost/thread/win32/mutex.hpp

@@ -161,6 +161,10 @@ namespace boost
             }
             void unlock()
             {
+                if(!locked())
+                {
+                    throw boost::lock_error();
+                }
                 m.unlock();
                 is_locked=false;
             }