Fix an off-by-one in role names

This was causing atk_role_for_name to return ATK_ROLE_LEVEL_BAR
when giving it an empty string, which strangely enough is something
that the lsb testsuite checks...

https://bugzilla.gnome.org/show_bug.cgi?id=704092
Merge branch 'master' of git://git.gnome.org/atk