Fixed #3848 -- Added more comprehensive checks to ImageField validation, checking for image truncation or corruption. Thanks to Andrew C <andrewc-djangotrac1@piffle.org> for the patch.

      git-svn-id: http://code.djangoproject.com/svn/django/trunk@6175 bcc190cf-cafb-0310-a4f2-bffc1f526a37