MDEV-4506: Parallel replication

Fix access of freed memory in debug builds. When deleting serial_rgi,
safe_mutex was trying to access current_thd, when that thd had just been
deleted (I hate all this current_thd and other magic thread local storage crap
used all over the code). Now delete the serial_rgi before the thd.

sql/slave.cc

@@ -4568,8 +4568,8 @@ log '%s' at position %s, relay log '%s' position: %s%s", RPL_LOG_NAME,
   serial_rgi->thd= rli->sql_driver_thd= 0;
   mysql_mutex_lock(&LOCK_thread_count);
   THD_CHECK_SENTRY(thd);
-  delete thd;
   delete serial_rgi;
+  delete thd;
   mysql_mutex_unlock(&LOCK_thread_count);
  /*
   Note: the order of the broadcast and unlock calls below (first broadcast, then unlock)