wbclient: ensure response struct is initialized

wbclient: ensure response struct is initialized

Prior to asking for a winbindd private pipe we need to initialize
response structure to deal with a possible response failure.

winbind_open_pipe_sock() issues two winbindd requests:
 - asks for interface version
 - asks for a private pipe

The first call returns interface version in a response structure (which
is a union). The second call might fail -- in this case response
structure will not be initialized or filled in with any information.

As result, if the second call failed, response structure will have data
from an interface string interpreted as a pointer to a string during
SAFE_FREE() at the end of the winbind_open_pipe_sock().

To avoid that, ensure response struct is initialized before asking for
a private pipe.

https://bugzilla.samba.org/show_bug.cgi?id=10596

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Thu May  8 04:24:53 CEST 2014 on sn-devel-104