s3-winbind: Do not delete an existing valid credential cache.

s3-winbind: Do not delete an existing valid credential cache.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=9994

Thanks to David Woodhouse <dwmw2@infradead.org>.

Reviewed-by: Günther Deschner <gd@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Mon Jul 15 12:48:46 CEST 2013 on sn-devel-104

(cherry picked from commit 0529b59fbe3f96509893fc4e93a75d6928b5a532)

Autobuild-User(v4-1-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-1-test): Thu Jul 18 21:56:40 CEST 2013 on sn-devel-104