Fixed #10472 -- Fixed a race condition in reverse URL resolving.

This only shows up in for reverse() (not forwards resolving), since that
path uses a globally shared resolver object. Based on a patch from
Travis Terry.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10037 bcc190cf-cafb-0310-a4f2-bffc1f526a37