Bug 12115: follow-up fix for tools/inventory's output_pref issues

This should correct issues with the output_prefs in the
compareinv2bar logic section of tools/inventory.pl
It is a follow up to bug 11253.

TEST PLAN
---------
NOTE: There should be no distinguishable difference before and
      after the patch. This corrects a works by accident bug.

Back up DB.
Create a small barcode file from books in a given branch.

Set I18N/L10N system preference of dateformat to mm/dd/yyyy.
Go to inventory/stocktaking tool.
Browse for the barcode file.
Click submit just below the 'Use a barcode file' section.
All looks fine in mm/dd/yyyy format.

Set I18N/L10N system preference of dateformat to yyyy-mm-dd.
Go to inventory/stocktaking tool.
Browse for the barcode file.
Click submit just below the 'Use a barcode file' section.
All looks fine in yyyy-mm-dd format.

Set I18N/L10N system preference of dateformat to dd/mm/yyyy.
Go to inventory/stocktaking tool.
Browse for the barcode file.
Click submit just below the 'Use a barcode file' section.
All looks fine in dd/mm/yyyy format.

Apply patch
Restore DB (so results should be comparable to the first result)
Repeat the three testing blocks above.
There should be no visible difference.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 40378d4a13e9d89bd84da90e9b6c0c5a237af1ef)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>