Cancel 'join room' action if 'log in' is clicked

or 'choose different server'

We canceled the deferred action in the MatrixChat SetMxId dialog
but not the one in roomview.

Fixes https://github.com/vector-im/riot-web/issues/4217

src/components/structures/RoomView.js

@@ -791,6 +791,9 @@ module.exports = React.createClass({
                     if (submitted) {
                         this.props.onRegistered(credentials);
                     } else {
+                        dis.dispatch({
+                            action: 'cancel_after_sync_prepared',
+                        });
                         dis.dispatch({
                             action: 'cancel_join',
                         });