MDEV-10653: Fix segfault in SHOW MASTER STATUS with NULL inuse_relaylog

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>

sql/rpl_parallel.cc

@@ -2538,7 +2538,9 @@ rpl_parallel::stop_during_until()
 bool
 rpl_parallel::workers_idle(Relay_log_info *rli)
 {
-  return rli->last_inuse_relaylog->queued_count ==
+  mysql_mutex_assert_owner(&rli->data_lock);
+  return !rli->last_inuse_relaylog ||
+    rli->last_inuse_relaylog->queued_count ==
     rli->last_inuse_relaylog->dequeued_count;
 }
 

sql/slave.cc

@@ -5369,19 +5369,25 @@ pthread_handler_t handle_slave_sql(void *arg)
   }
   else
     rli->gtid_skip_flag = GTID_SKIP_NOT;
+  mysql_mutex_lock(&rli->data_lock);
   if (init_relay_log_pos(rli,
                          rli->group_relay_log_name,
                          rli->group_relay_log_pos,
-                         1 /*need data lock*/, &errmsg,
+                         0 /*need data lock*/, &errmsg,
                          1 /*look for a description_event*/))
   { 
     rli->report(ERROR_LEVEL, ER_SLAVE_FATAL_ERROR, NULL,
                 "Error initializing relay log position: %s", errmsg);
+    mysql_mutex_unlock(&rli->data_lock);
     goto err_before_start;
   }
   rli->reset_inuse_relaylog();
   if (rli->alloc_inuse_relaylog(rli->group_relay_log_name))
+  {
+    mysql_mutex_unlock(&rli->data_lock);
     goto err_before_start;
+  }
+  mysql_mutex_unlock(&rli->data_lock);
 
   strcpy(rli->future_event_master_log_name, rli->group_master_log_name);
   THD_CHECK_SENTRY(thd);