Fix possible crash when clicking on Filenodes with a line number

If the path() set on that node can not be opened, then the editor
is 0 and thus there is a crash.

Task-number: QBS-367
Change-Id: I8887a3bb84e19292d97357df49ef988d74bf6739
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>