Avoiding a theoretically possible crash (pthread_mutex_lock(0)) which could (at least in POSIX Threads books)
happen on SMP machines, when a thread is going to wait on a condition and it is KILLed at the
same time.
Cleaning code a bit by adding a test in enter_cond() that we have the mutex (was already the case in all places
where it's called except one which is fixed here).