[afsql] fixed crash on AIX where zero-initializing a GStaticMutex renders the mutex ...

[afsql] fixed crash on AIX where zero-initializing a GStaticMutex renders the mutex useless (fixes: #16306)

	G_STATIC_MUTEX_INIT is not equivalent to zero initializing the
	mutex on AIX. self->queue_lock has only been zero-initialized
	and this causes races on the LogQueue instance used to transfer
	messages between the main and the SQL thread.