DETCI: thread pool fixed possible hang

When a thread pool is opend, the number of awake threads should not be
set to zero. If a call to tpool_init() is followed by a call to
tpool_queue_open() and followed by a call to tpool_add_work() while
all threads are still waiting for the mutex queue_lock, the number of
awake thread will go below zero at line tpool.cc:335

The fix in #20 was not enough.