pthread: Fix deadlock during thread initialization

Sometimes, if pthread_create() failed, then pthread_cond_wait() could
accidentally be called in the worker threads after the uninit function
had already called pthread_cond_broadcast(), leading to a deadlock.

Don't call pthread_cond_wait() if c->done is set.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>