sysupdate: fix noop assert

Reported by qarmin (Rafał Mikrut).

src/sysupdate/sysupdate-pattern.c

@@ -420,7 +420,7 @@ int pattern_match(const char *pattern, const char *s, InstanceMetadata *ret) {
                 }
 
                 default:
-                        assert_se("unexpected pattern element");
+                        assert_not_reached();
                 }
 
                 p = n;