LP:482100 - Valgrind warning and garbled output from innodb_file_format_name_update()

Using this test case (simplified from innodb_xtradb_bug317074.test):

-----------------------------------------------------------------------
--source include/have_innodb.inc

SET @old_innodb_file_format=@@innodb_file_format;
SET @old_innodb_file_format_check=@@innodb_file_format_check;

SET GLOBAL innodb_file_format=@old_innodb_file_format;
SET GLOBAL innodb_file_format_check=@old_innodb_file_format_check;
-----------------------------------------------------------------------

perl mysql-test-run.pl --valgrind innodb-kn

I get this warning and stack trace:

==27371== Conditional jump or move depends on uninitialised value(s)
==27371== at 0xA3D759: innobase_file_format_name_lookup(char const*) (ha_innodb.cc:9568)
==27371== by 0xA3D941: innodb_file_format_name_update(THD*, st_mysql_sys_var*, void*, void const*) (ha_innodb.cc:9710)
==27371== by 0x86B3D4: sys_var_pluginvar::update(THD*, set_var*) (sql_plugin.cc:2802)
==27371== by 0x6903DB: set_var::update(THD*) (set_var.cc:3674)
==27371== by 0x697535: sql_set_variables(THD*, List<set_var_base>*) (set_var.cc:3549)
==27371== by 0x687125: mysql_execute_command(THD*) (sql_parse.cc:3495)
==27371== by 0x68C1EC: mysql_parse(THD*, char const*, unsigned, char const**) (sql_parse.cc:5983)
==27371== by 0x68CFB1: dispatch_command(enum_server_command, THD*, char*, unsigned) (sql_parse.cc:1227)
==27371== by 0x68E398: do_command(THD*) (sql_parse.cc:866)
==27371== by 0x679FB2: handle_one_connection (sql_connect.cc:1130)
==27371== by 0x50463F6: start_thread (pthread_create.c:297)
==27371== by 0x6026B4C: clone (in /usr/lib/debug/libc-2.7.so)

I also get a spurious warning due to reading garbage data in
innobase_file_format_name_lookup().

I found bug 461155, which may be related. But I checked the fix committed as
rev. 112 of lp:~percona-dev/percona-xtradb/alignment-for-5.1.40. This fix
seems to just rewrite the test cases, so I'm thinking that if it is related it
just hides the problem rather than fix it.

I will debug to investigate.