Bug 18651: [QA Follow-up] Fix the MAX(issue_id) calculation

Bug 18651: [QA Follow-up] Fix the MAX(issue_id) calculation

Found this by inserting the same issue_id in old_issues before checkin:
The call to ->search( )->get_column is in scalar context and will
return the number of results, i.e. always 1.
If you have an issue_id 2 in old_issues, it will crash:
    DBIx::Class::Storage::DBI::_dbh_execute(): Duplicate entry '2' for key 'PRIMARY'

The fix is fairly simple: Put get_column in list context and pick the first
array entry.
NOTE: Using DBIx's get_column()->max here might look simpler here.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>