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).


sql/log.cc:
  safe_mutex_assert_owner() is now in THD::enter_cond()
sql/slave.cc:
  lock mutex before waiting on condition.
sql/sql_class.cc:
  THD::awake(): before locking the mutex, let's test it's not zero;
  in theory indeed, the killer thread may see current_cond non-zero and current_mutex zero
  (order of assignments is not guaranteed by POSIX).
  A comment noting that there is still a small chance a KILL does not work and needs
  being re-issued.
sql/sql_class.h:
  Assert in enter_cond() that we have the mutex.
  It is already the case in all places where we call enter_cond(), so better ensure it there.