libcli/auth: let msrpc_parse() return talloc'ed empty strings

libcli/auth: let msrpc_parse() return talloc'ed empty strings

This make it more predictable for the callers.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11912
BUG: https://bugzilla.redhat.com/show_bug.cgi?id=1334356
BUG: https://launchpad.net/bugs/1578576

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Mon May  9 22:27:21 CEST 2016 on sn-devel-144

(cherry picked from commit 58a83236294117d32d9883ac3024f81fa1730a87)

Autobuild-User(v4-2-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-2-test): Tue May 17 13:47:41 CEST 2016 on sn-devel-104