Utility user memory leak

Similar to bug 1166638, seen in 5.7 branch at least:

$ ./mtr --debug-server percona_utility_user --valgrind
...
42 bytes in 1 blocks are definitely lost in loss record 4 of 7
   at 0x1960FB4: my_malloc (my_malloc.c:62)
   by 0xEFF3AA: acl_init_utility_user(bool) (sql_auth_cache.cc:1543)
   by 0xF00D78: acl_load(THD*, TABLE_LIST*) (sql_auth_cache.cc:2062)
   by 0xF01ACF: acl_reload(THD*) (sql_auth_cache.cc:2314)
   by 0xEFEF5A: acl_init(bool) (sql_auth_cache.cc:1436)
   by 0xED138D: mysqld_main(int, char**) (mysqld.cc:5100)
   by 0xEC78B5: main (main.cc:25)

42 bytes in 1 blocks are definitely lost in loss record 5 of 7
   at 0x1960FB4: my_malloc (my_malloc.c:62)
   by 0xEFF3AA: acl_init_utility_user(bool) (sql_auth_cache.cc:1543)
   by 0xF00D78: acl_load(THD*, TABLE_LIST*) (sql_auth_cache.cc:2062)
   by 0xF01ACF: acl_reload(THD*) (sql_auth_cache.cc:2314)
   by 0x165DB9D: reload_acl_and_cache(THD*, unsigned long, TABLE_LIST*, int*) (sql_reload.cc:84)
   by 0x1616F10: mysql_execute_command(THD*, bool) (sql_parse.cc:4336)
   by 0x161B204: mysql_parse(THD*, Parser_state*) (sql_parse.cc:5786)
   by 0x160F420: dispatch_command(THD*, COM_DATA const*, enum_server_command) (sql_parse.cc:1445)
   by 0x160E2A7: do_command(THD*) (sql_parse.cc:1008)
   by 0x175D6C2: handle_connection (connection_handler_per_thread.cc:313)
   by 0x1E7DA80: pfs_spawn_thread (pfs.cc:2192)
   by 0x5D306A9: start_thread (pthread_create.c:333)
   by 0x68EEEEC: clone (clone.S:109)

42 bytes in 1 blocks are definitely lost in loss record 6 of 7
   at 0x1960FB4: my_malloc (my_malloc.c:62)
   by 0xEFF3AA: acl_init_utility_user(bool) (sql_auth_cache.cc:1543)
   by 0xF00D78: acl_load(THD*, TABLE_LIST*) (sql_auth_cache.cc:2062)
   by 0xF01ACF: acl_reload(THD*) (sql_auth_cache.cc:2314)
   by 0xF1D955: acl_end_trans_and_close_tables(THD*, bool) (sql_user_table.cc:324)
   by 0xF174CD: mysql_revoke_all(THD*, List<st_lex_user>&) (sql_authorization.cc:3508)
   by 0x16165CD: mysql_execute_command(THD*, bool) (sql_parse.cc:4159)
   by 0x161B204: mysql_parse(THD*, Parser_state*) (sql_parse.cc:5786)
   by 0x160F420: dispatch_command(THD*, COM_DATA const*, enum_server_command) (sql_parse.cc:1445)
   by 0x160E2A7: do_command(THD*) (sql_parse.cc:1008)
   by 0x175D6C2: handle_connection (connection_handler_per_thread.cc:313)
   by 0x1E7DA80: pfs_spawn_thread (pfs.cc:2192)
   by 0x5D306A9: start_thread (pthread_create.c:333)
   by 0x68EEEEC: clone (clone.S:109)