Fix null pointer exception when deleting a run from the web UI.

Previously, when selecting a run and clicking 'Delete' (from the View
Results page) resulted in an exception being shown in the page
(although the run data did get deleted from disk). If multiple runs
had been selected, only the first one was actually deleted from disk.
Merge pull request #61 from jvirkki/jyri-fix-delete-run

Fix null pointer exception when deleting a run from the web UI.