userdb: Fix return value of groupdb_by_name()

The commit 7419291670 ("userdb: move UserDBMatch handling from userdbctl
into generic userdb code to allow it to be done server side")
unintentionally passes return value from group_record_match() as its
return value and thus diverges from other search functions that return 0
on success. Align that by returning 0 instead of 1, all existing callers
are invariant to this change.

src/shared/userdb.c

@@ -1356,7 +1356,7 @@ int groupdb_by_name(const char *name, const UserDBMatch *match, UserDBFlags flag
         if (ret)
                 *ret = TAKE_PTR(gr);
 
-        return r;
+        return 0;
 }
 
 static int groupdb_by_gid_fallbacks(