[1.3.X] Fixed #16201 -- Ensure that requests with Content-Length=0 don't break...

[1.3.X] Fixed #16201 -- Ensure that requests with Content-Length=0 don't break the multipart parser. Thanks to albsen for the report and patch

Backport of r16353 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@16676 bcc190cf-cafb-0310-a4f2-bffc1f526a37