ldap: make sure realm is set

In general the canonical principal will be only set in the cache after a
successful authentication because in general it is not know what the
canonical principal might be.

For Active Directory it is known that the canonical principal is build
with the sAMAccountName attribute and the Kerberos realm which is used
in the patch "AD: Construct UPN from the sAMAccountName" (7a27e53). If
'id_provider = ldap' is used to access Active Directory the realm might
not be set in the internal domain data and as a result a wrong principal
might be created. This patch makes sure the realm is set before creating
the canonical principal.

src/providers/ldap/sdap_async_users.c

@@ -204,7 +204,7 @@ int sdap_save_user(TALLOC_CTX *memctx,
     size_t c;
     char *p1;
     char *p2;
-    char *new_upn;
+    char *new_upn = NULL;
     bool is_posix = true;
 
     DEBUG(SSSDBG_TRACE_FUNC, "Save user\n");
@@ -278,8 +278,10 @@ int sdap_save_user(TALLOC_CTX *memctx,
                                    &samaccountname);
         if (ret == EOK) {
             ret = ENOENT;
-            new_upn = talloc_asprintf(memctx, "%s@%s", samaccountname,
-                                      dom->realm);
+            if (dom->realm != NULL) {
+                new_upn = talloc_asprintf(memctx, "%s@%s", samaccountname,
+                                          dom->realm);
+            }
             if (new_upn != NULL){
                 ret = sysdb_attrs_add_string(user_attrs,
                                              SYSDB_CANONICAL_UPN, new_upn);