sysusers: log new group name on write failure

When adding new groups, putgrent_with_members() is called with the newly
constructed group n, but the error path logs gr->gr_name. gr belongs to
the
earlier loop over the existing group file and may be NULL after EOF.

Log n.gr_name instead.

Found by Linux Verification Center (linuxtesting.org) with Svace.

src/sysusers/sysusers.c

@@ -821,7 +821,7 @@ static int write_temporary_group(
                 r = putgrent_with_members(c, &n, group);
                 if (r < 0)
                         return log_error_errno(r, "Failed to add new group \"%s\" to temporary group file: %m",
-                                               gr->gr_name);
+                                               n.gr_name);
 
                 group_changed = true;
         }