[BUGFIX] Error when editing a record with additionalPreviewLanguage and an deleted l18n_parent

The behaviour that causes the error (translated elements are not deleted
when deleting parent) is fixed in recent TYPO3 versions; it is better to
do that additional check anyways (for old installations).

Change-Id: Id51afff4349a473f291ef50631b519942f45be13
Resolves: #17910
Releases: 4.4, 4.5, 4.6
Reviewed-on: http://review.typo3.org/4734
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Björn Pedersen
Reviewed-by: Stefan Neufeind
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf