Cygwin: acls: allow converting empty acl to text

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/release/3.0.4

@@ -14,3 +14,6 @@ Bug Fixes
 
 - Fix access to process list
   Addresses: https://cygwin.com/ml/cygwin/2019-03/msg00253.html
+
+- Fix acl_to_text/acl_to_any_text returning EINVAL on empty acl
+  Addresses: https://cygwin.com/ml/cygwin/2019-03/msg00157.html

winsup/cygwin/sec_acl.cc

@@ -1638,8 +1638,8 @@ char *
 __acltotext (aclent_t *aclbufp, int aclcnt, const char *prefix, char separator,
 	     int options)
 {
-  if (!aclbufp || aclcnt < 1 || aclcnt > MAX_ACL_ENTRIES
-      || aclsort32 (aclcnt, 0, aclbufp))
+  if (!aclbufp || aclcnt < 0 || aclcnt > MAX_ACL_ENTRIES
+      || (aclcnt > 0 && aclsort32 (aclcnt, 0, aclbufp)))
     {
       set_errno (EINVAL);
       return NULL;