This tracks the locks manually, which is a lot slower but compared
to the existing vlc_assert_locked():
 - The performance is not a major problem in debug builds.
 - Assertion can be made that a mutex is _not_ locked.
 - The state of a static mutex can be asserted.
 - The implementation is not POSIX-specific.
 - The pointed to the asserted lock is properly const.
 - The implementation does not trip valgrind.
 - The implementation will not trip sanitizers.

Also the implementation is still lock-free and thread-safe.