[BUGFIX] Delete file with missing processed file raises alert

When a BE User deletes a file in the file module all file references
as well as processed files are removed, too. If then aprocessed files
is missing for some reason an alert is displayed in the backend module:

Could not delete file "foo.jpg". Write-permission problem?

The patch fixes the situation by adding a check whether before deleting.

Change-Id: I228624a3470340382950d3f54d8b428bfe831715
Releases: 6.2
Resolves: #55428
Reviewed-on: https://review.typo3.org/27123
Reviewed-by: Stefan Froemken
Tested-by: Stefan Froemken
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter