pop3-login: Code cleanup - Make sure destroying client stops processing.

It would currently do it in any case, but this makes it clearer.

src/pop3-login/client.c

@@ -138,6 +138,7 @@ static void pop3_client_input(struct client *client)
 				"Too many invalid bad commands.");
 			client_destroy(client,
 				       "Disconnected: Too many bad commands");
+			break;
 		}
 	}