Fixed #13670 -- Comparisons with the spatial adapter won't blow up in some...

Fixed #13670 -- Comparisons with the spatial adapter won't blow up in some corner cases.  Thanks, milosu for the bug report and jpaulett for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16757 bcc190cf-cafb-0310-a4f2-bffc1f526a37