Fixed #17542 -- Gracefully handle errors when checking if the values of a...

Fixed #17542 -- Gracefully handle errors when checking if the values of a SelectDateWidget has changed if it's not required. Thanks, pigletto.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17436 bcc190cf-cafb-0310-a4f2-bffc1f526a37