[sanity] qb_log_fini should assuredly (by the means of a guard) only be called from whatever thread but the logging worker

...which could possibly lead to deadlock or other funny moments.

This is to prevent situation like `SIGSEGV` handler, deliberately and
acceptably calling `qb_log_fini` (like corosync does) being unknowingly
arranged also for the logging worker should the SIGSEGV be raised
in its context:

"A signal may be generated (and thus pending) for a process as
a whole (e.g., when sent using kill(2)) or for a specific thread
(e.g., certain signals, such as SIGSEGV and SIGFPE, generated as
a consequence of executing a specific machine-language instruction
are thread directed, as are signals targeted at a specific thread
using pthread_kill(3)). A process-directed signal may be delivered
to any one of the threads that does not currently have the signal
blocked. If more than one of the threads has the signal unblocked,
then the kernel chooses an arbitrary thread to which to deliver the
signal." [signal(7)]

Consider, for example, that the main thread performs, sequentially,
logging to blackbox, which could possibly result in another nasty race.