Fixed #11414 -- Made sure the calendar and clock popup windows in the admin...

Fixed #11414 -- Made sure the calendar and clock popup windows in the admin don't have a negative vertical position. Thanks uipko for the report and fix.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15143 bcc190cf-cafb-0310-a4f2-bffc1f526a37