Do not schedule navigation to invalid URL

Fix an assert triggered by navigation to a fragment URL of an unset
URL. The code was assuming the string given was a valid encoded
URL, so we shouldn't continue if the completed URL is invalid.

Task-number: QTBUG-38546
Change-Id: I377b09814150ae9926de95efe3f3e2e750145c4a
Reviewed-by: Michael Bruning <michael.bruning@digia.com>