MDEV-5363: Make parallel replication waits killable

Fix locking bug / race introduced by previous patch: If we
bail out of next_event() due to kill, there was a small window
where we could exit without rli->data_lock held, causing an
unlock-twice bug.

sql/slave.cc

@@ -6181,6 +6181,17 @@ static Log_event* next_event(rpl_group_info *rgi, ulonglong *event_size)
           DBUG_RETURN(ev);
         }
 
+        /*
+          We have to check sql_slave_killed() here an extra time.
+          Otherwise we may miss a wakeup, since last check was done
+          without holding LOCK_log.
+        */
+        if (sql_slave_killed(rgi))
+        {
+          mysql_mutex_unlock(log_lock);
+          break;
+        }
+
         /*
           We can, and should release data_lock while we are waiting for
           update. If we do not, show slave status will block
@@ -6241,17 +6252,6 @@ static Log_event* next_event(rpl_group_info *rgi, ulonglong *event_size)
           rli->ignore_log_space_limit= true;
         }
 
-        /*
-          We have to check sql_slave_killed() here an extra time.
-          Otherwise we may miss a wakeup, since last check was done
-          without holding LOCK_log.
-        */
-        if (sql_slave_killed(rgi))
-        {
-          mysql_mutex_unlock(log_lock);
-          break;
-        }
-
         /*
           If the I/O thread is blocked, unblock it.  Ok to broadcast
           after unlock, because the mutex is only destroyed in