Fix crash #1 when filtering items

When filtering items it was possible that the current index got
an invalid value which resulted in accessing the URL of a
null-KFileItem.

There is still one (general) open issue in KFileItemModelRolesUpdater
(crash #2) where a KFileItem that is already null gets read.
It is not really related to filtering but can be triggered
quite easy when filtering huge directories with enabled previews.