s3:lib: Make sure struct security_ace is fully initialized

s3:lib: Make sure struct security_ace is fully initialized

Found by Covscan.

"Error: UNINIT (CWE-457):
samba-4.20.0rc2/source3/lib/sharesec.c:275: var_decl: Declaring variable ""ace"" without initializer.
samba-4.20.0rc2/source3/lib/sharesec.c:285: uninit_use_in_call: Using uninitialized value ""ace"". Field ""ace.object"" is uninitialized when calling ""make_sec_acl"".
  283|   	init_sec_ace(&ace, &global_sid_World, SEC_ACE_TYPE_ACCESS_ALLOWED, sa, 0);
  284|
  285|-> 	if ((psa = make_sec_acl(ctx, NT4_ACL_REVISION, 1, &ace)) != NULL) {
  286|   		psd = make_sec_desc(ctx, SECURITY_DESCRIPTOR_REVISION_1,
  287|   				    SEC_DESC_SELF_RELATIVE, NULL, NULL, NULL,"

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