Char chunking doesn't quote column name

received this error from pt-table-sync:

 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'key)) FROM `<database>`.`<column>` ORDER BY `key`' at line 1 [for Statement "SELECT MAX(LENGTH(key)) FROM `<database>`.`<table>` ORDER BY `key`"] at line 3316 while doing <database>.<column> on <host>

the paramater to the 'LENGTH' function needs quoting.

here is a patch:

*** modified/pt-table-sync 2012-03-28 11:00:55.000000000 -0500
--- original/pt-table-sync 2012-03-07 17:47:53.000000000 -0600
*************** sub _chunk_char {
*** 3309,3315 ****
     PTDEBUG && _d("Base", $base, "chars:", @chars);

! $sql = "SELECT MAX(LENGTH(`$chunk_col`)) FROM $db_tbl "
          . ($args{where} ? "WHERE $args{where} " : "")
          . "ORDER BY `$chunk_col`";
     PTDEBUG && _d($dbh, $sql);
--- 3309,3315 ----
     PTDEBUG && _d("Base", $base, "chars:", @chars);

! $sql = "SELECT MAX(LENGTH($chunk_col)) FROM $db_tbl "
          . ($args{where} ? "WHERE $args{where} " : "")
          . "ORDER BY `$chunk_col`";
     PTDEBUG && _d($dbh, $sql);