Avoid propagating Undo messages innecesarily.

activitypub.c

@@ -468,6 +468,11 @@ int is_msg_for_me(snac *snac, const xs_dict *c_msg)
             return 0;
     }
 
+    /* if it's an Undo, it must be from someone we follow */
+    if (strcmp(type, "Undo") == 0) {
+        return following_check(snac, xs_dict_get(c_msg, "actor"));
+    }
+
     /* if it's not a Create or Update, allow */
     if (!xs_match(type, "Create|Update"))
         return 1;