The previous patch for MDEV-10653 changes the rpl_parallel::workers_idle()
function to use Relay_log_info::last_inuse_relaylog to check for idle
workers. But the code was missing a NULL check. Also, there was one place
during SQL slave thread start which was missing mutex synchronisation when
updating inuse_relaylog.
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>