Fixed #17014 - added protection against infinite recursion.

Thanks to akaariai for the report and tests.

No tests have been added, since unittests for termination are basically
impossible, and the failure condition will take down the developer's machine
in this case. It has been tested against the cases in #17014.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16940 bcc190cf-cafb-0310-a4f2-bffc1f526a37