s3:rpc_server: Make sure struct security_ace is initialized

s3:rpc_server: Make sure struct security_ace is initialized

Found by Covscan.

"Error: UNINIT (CWE-457):
samba-4.20.0rc2/source3/rpc_server/samr/srv_samr_nt.c:235: var_decl: Declaring variable ""ace"" without initializer.
samba-4.20.0rc2/source3/rpc_server/samr/srv_samr_nt.c:269: uninit_use_in_call: Using uninitialized value ""*ace"". Field ""ace->object"" is uninitialized when calling ""make_sec_acl"".
  267|   	/* create the security descriptor */
  268|
  269|-> 	if ((psa = make_sec_acl(ctx, NT4_ACL_REVISION, i, ace)) == NULL)
  270|   		return NT_STATUS_NO_MEMORY;
  271|"

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Signed-off-by: Martin Schwenke <mschwenke@ddn.com>