Cocoa: fix a crash in QCocoaFileDialogHelper

Current directory can't be nil.

Task-number: QTBUG-34561
Change-Id: I58c84da4928bd9081c916b6e5541bb86cf75ed47
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>