MDEV-18993 The keep-alive connection (set spider_conn_recycle_mode = 1) in spider would cause cash in MariaDB (#1269)

Fix the following valgrind error.

==94390== Thread 29:
==94390== Invalid read of size 8
==94390== at 0x78389D: thd_increment_bytes_sent (sql_class.cc:4265)
==94390== by 0xC8EC46: net_real_write (net_serv.cc:730)
==94390== by 0xC8E0C8: net_flush (net_serv.cc:383)
==94390== by 0xC8E4D0: net_write_command (net_serv.cc:521)
==94390== by 0xADCE61: cli_advanced_command (client.c:468)
==94390== by 0xAE3CAF: mysql_close_slow_part (client.c:3671)
==94390== by 0xAE3D28: mysql_close (client.c:3683)
==94390== by 0x149E69A8: spider_db_mbase::disconnect() (spd_db_mysql.cc:2217)
==94390== by 0x1491EA26: spider_db_disconnect(st_spider_conn*) (spd_db_conn.cc:297)
==94390== by 0x14948EBE: spider_free_conn_alloc(st_spider_conn*) (spd_conn.cc:196)
==94390== by 0x1494B26A: spider_free_conn(st_spider_conn*) (spd_conn.cc:1251)
==94390== by 0x1494941F: spider_free_conn_from_trx(st_spider_transaction*, st_spider_conn*, bool, bool, int*) (spd_conn.cc:315)
==94390== Address 0x1f0e0990 is 4,832 bytes inside a block of size 25,728 free'd
==94390== at 0x4C2ACBD: free (vg_replace_malloc.c:530)
==94390== by 0x13F5545: my_free (my_malloc.c:222)
==94390== by 0x6C75B7: ilink::operator delete(void*, unsigned long) (sql_list.h:618)
==94390== by 0x77B9F6: THD::~THD() (sql_class.cc:1724)
==94390== by 0x1494FCE0: spider_bg_conn_action(void*) (spd_conn.cc:2580)
==94390== by 0x4E3DDD4: start_thread (in /usr/lib64/libpthread-2.17.so)
==94390== by 0x5FBFEAC: clone (in /usr/lib64/libc-2.17.so)
==94390== Block was alloc'd at
==94390== at 0x4C29BC3: malloc (vg_replace_malloc.c:299)
==94390== by 0x13F4DFA: my_malloc (my_malloc.c:101)
==94390== by 0x1491CF06: ilink::operator new(unsigned long) (sql_list.h:614)
==94390== by 0x1494F7FD: spider_bg_conn_action(void*) (spd_conn.cc:2501)
==94390== by 0x4E3DDD4: start_thread (in /usr/lib64/libpthread-2.17.so)
==94390== by 0x5FBFEAC: clone (in /usr/lib64/libc-2.17.so)
==94390== Invalid write of size 8
==94390== at 0x7838AF: thd_increment_bytes_sent (sql_class.cc:4265)
==94390== by 0xC8EC46: net_real_write (net_serv.cc:730)
==94390== by 0xC8E0C8: net_flush (net_serv.cc:383)
==94390== by 0xC8E4D0: net_write_command (net_serv.cc:521)
==94390== by 0xADCE61: cli_advanced_command (client.c:468)
==94390== by 0xAE3CAF: mysql_close_slow_part (client.c:3671)
==94390== by 0xAE3D28: mysql_close (client.c:3683)
==94390== by 0x149E69A8: spider_db_mbase::disconnect() (spd_db_mysql.cc:2217)
==94390== by 0x1491EA26: spider_db_disconnect(st_spider_conn*) (spd_db_conn.cc:297)
==94390== by 0x14948EBE: spider_free_conn_alloc(st_spider_conn*) (spd_conn.cc:196)
==94390== by 0x1494B26A: spider_free_conn(st_spider_conn*) (spd_conn.cc:1251)
==94390== by 0x1494941F: spider_free_conn_from_trx(st_spider_transaction*, st_spider_conn*, bool, bool, int*) (spd_conn.cc:315)
==94390== Address 0x1f0e0990 is 4,832 bytes inside a block of size 25,728 free'd
==94390== at 0x4C2ACBD: free (vg_replace_malloc.c:530)
==94390== by 0x13F5545: my_free (my_malloc.c:222)
==94390== by 0x6C75B7: ilink::operator delete(void*, unsigned long) (sql_list.h:618)
==94390== by 0x77B9F6: THD::~THD() (sql_class.cc:1724)
==94390== by 0x1494FCE0: spider_bg_conn_action(void*) (spd_conn.cc:2580)
==94390== by 0x4E3DDD4: start_thread (in /usr/lib64/libpthread-2.17.so)
==94390== by 0x5FBFEAC: clone (in /usr/lib64/libc-2.17.so)
==94390== Block was alloc'd at
==94390== at 0x4C29BC3: malloc (vg_replace_malloc.c:299)
==94390== by 0x13F4DFA: my_malloc (my_malloc.c:101)
==94390== by 0x1491CF06: ilink::operator new(unsigned long) (sql_list.h:614)
==94390== by 0x1494F7FD: spider_bg_conn_action(void*) (spd_conn.cc:2501)
==94390== by 0x4E3DDD4: start_thread (in /usr/lib64/libpthread-2.17.so)
==94390== by 0x5FBFEAC: clone (in /usr/lib64/libc-2.17.so)