[1.2.X] Fixed #11414 -- Made sure the calendar and clock popup windows in the...

[1.2.X] 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.

Backport of [15143] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15180 bcc190cf-cafb-0310-a4f2-bffc1f526a37