policy: minor fix when having multiple default routes with metric MAXUINT32

The case of having a metric MAXUINT32 is special, because in face of
multiple default routes with the same metric, NMDefaultRouteManager
cannot reduce the effective metric (because there is no lower priority
value).

This case works already correct, just when adding such a default route,
ensure that we add it to the *first* entry.

Signed-off-by: Thomas Haller <thaller@redhat.com>

src/nm-default-route-manager.c

@@ -161,7 +161,8 @@ _platform_route_sync_add (const VTableIP *vtable, NMDefaultRouteManager *self, g
 
 		if (e->synced) {
 			g_assert (!entry || metric == G_MAXUINT32);
-			entry = e;
+			if (!entry)
+				entry = e;
 		} else
 			has_unsynced_entry = TRUE;
 	}