Coverity has generated a number of 'Data race condition' and 'Double
lock' false positives. A lot of these seem to be caused by the NULL
guard in tc_mutex_unlock() not being paired with a NULL guard in
tc_mutex_lock(). This PR adds a NULL guard to tc_mutex_lock().
It should be noted, that on Linux at least, passing NULL to
tc_mutex_lock() causes a segfault. We clearly aren't doing this at the
moment, or we'd know about it. A log message is generated if a NULL
call is made, rather than failing silently.
matt335672
67fbccc539