Only break if Bob has requested amounts already

We don't want Bob to be able to crash us by sending an out of
order message. Only break if Bob has not requested amounts.

swap/src/alice.rs

@@ -51,10 +51,13 @@ pub async fn swap(
                 swarm.send_amounts(channel, p);
             }
             OutEvent::Message0(msg) => {
-                debug!("Got message0 from Bob");
-                // TODO: Do this in a more Rusty/functional way.
-                message0 = msg;
-                break;
+                // We don't want Bob to be able to crash us by sending an out of
+                // order message. Keep looping if Bob has not requested amounts.
+                if last_amounts.is_some() {
+                    // TODO: We should verify the amounts and notify Bob if they have changed.
+                    message0 = msg;
+                    break;
+                }
             }
             other => panic!("Unexpected event: {:?}", other),
         };