Backport the fix f214d365 to 10.0

   Author: Sergei Golubchik <serg@mariadb.org>
   Date:   Tue Apr 17 00:44:34 2018 +0200

    ASAN error in is_stat_table()

    don't memcmp beyond the first argument's end

    Also: use my_strcasecmp(table_alias_charset), like elsewhere, not memcmp