MDEV-29605 Reset queued ping info of all spider connections associated with a closed spider handler

A spider_conn may outlive its associated ha_spider (in the field
queued_ping_spider) used for connecting to and pinging the data
node (a call to spider_db_ping(), guarded by the boolean field
queued_ping). In a call to ha_spider::close() (which is often preceded
with the deletion of the ha_spider itself), many cleanups happen,
including freeing the associated spider_share, which is used by the
spider_conn in spider_db_ping. Therefore it is necessary to reset both
the queued_ping_spider and queued_ping fields, so that any further
spider interaction with the data node will not trigger the call using
the ha_spider including its freed spider_share.

Also out of caution added an assert and internal error in case a
connection has not been established (the db_conn field of type
MYSQL * is NULL), and attempt to connect is skipped because both
queued_connect and queued_ping are false. Note that this unlikely (if
not impossible) scenario would not be a regression caused by this
change, as it strictly falls under the scenario of this bug.

storage/spider/spd_db_mysql.cc

@@ -2100,6 +2100,10 @@ int spider_db_mbase::exec_query(
     general_log_write(current_thd, COM_QUERY, tmp_query_str.ptr(),
       tmp_query_str.length());
   }
+  /* There should be a live connection to the data node */
+  DBUG_ASSERT(db_conn);
+  if (!db_conn)
+    DBUG_RETURN(HA_ERR_INTERNAL_ERROR);
   if (!spider_param_dry_access())
   {
     error_num = mysql_real_query(db_conn, query, length);