Fixed #12712 -- Corrected a problem with synchronizing that prevented m2m...

Fixed #12712 -- Corrected a problem with synchronizing that prevented m2m tables from being created under certain circumstances. Thanks to IonelMaries for the report, and Alex Gaynor for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12597 bcc190cf-cafb-0310-a4f2-bffc1f526a37