Fix setting guest nicks

The "nick" attribute was set with itself instead of with the
"internalNick" variable that holds the value received in the
"nickChanged" event.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

app/src/main/java/com/nextcloud/talk/webrtc/MagicPeerConnectionWrapper.java

@@ -271,7 +271,7 @@ public class MagicPeerConnectionWrapper {
                     if (dataChannelMessage.getPayload() instanceof String) {
                         internalNick = (String) dataChannelMessage.getPayload();
                         if (!internalNick.equals(nick)) {
-                            setNick(nick);
+                            setNick(internalNick);
                             EventBus.getDefault().post(new PeerConnectionEvent(PeerConnectionEvent.PeerConnectionEventType
                                     .NICK_CHANGE, sessionId, getNick(), null, videoStreamType));
                         }