main.explain_json, main.explain_json_innodb fail in buildbot with valgrind

==24018== Thread 21:

==24018== Conditional jump or move depends on uninitialised value(s)

==24018==    at 0x72BA3D: Explain_node::print_explain_json_for_children(Explain_query*, Json_writer*, bool) (sql_explain.cc:612)

==24018==    by 0x72EAD2: Explain_basic_join::print_explain_json_interns(Explain_query*, Json_writer*, bool) (sql_explain.cc:800)

==24018==    by 0x72ED7C: Explain_select::print_explain_json(Explain_query*, Json_writer*, bool) (sql_explain.cc:765)

==24018==    by 0x72F15B: Explain_query::print_explain_json(select_result_sink*, bool) (sql_explain.cc:209)

==24018==    by 0x5F7806: execute_sqlcom_select(THD*, TABLE_LIST*) (sql_parse.cc:5733)

==24018==    by 0x5FCE0B: mysql_execute_command(THD*) (sql_parse.cc:2926)

==24018==    by 0x603E69: mysql_parse(THD*, char*, unsigned int, Parser_state*) (sql_parse.cc:7165)

==24018==    by 0x607091: dispatch_command(enum_server_command, THD*, char*, unsigned int) (sql_parse.cc:1462)

==24018==    by 0x60761A: do_command(THD*) (sql_parse.cc:1090)

==24018==    by 0x6FAF1B: do_handle_one_connection(THD*) (sql_connect.cc:1347)

==24018==    by 0x6FB0C4: handle_one_connection (sql_connect.cc:1258)

==24018==    by 0x9C7E86: pfs_spawn_thread (pfs.cc:1860)

==24018==    by 0x4E3506F: start_thread (in /lib64/libpthread-2.9.so)

==24018==    by 0x62F913C: clone (in /lib64/libc-2.9.so)

^ Found warnings in /dev/shm/var_auto_Axsl/3/log/mysqld.1.err

ok