Comment 1 for bug 729497

Tested this logic change and it seems to work just fine. The checking for whether the binary exists and is valid is not as exhaustive as in the if($option_apply_log) section, but at least it makes the option useable.

I'm finding this necessary to avoid some crashes when performing apply-log using xtrabackup_51 binary (result of MySQL bug in InnoDB that it is linked against).