Fixed #17877 -- Ensured that extra WHERE clauses get correctly ANDed when they contain OR operations. Thanks to Marek Brzóska for the report, to eleather for the test case and to Adrien Lemaire for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17880 bcc190cf-cafb-0310-a4f2-bffc1f526a37