video_rtp_session: check rx thread for null

Change-Id: Iecdceb428e0c174883c94754444c0046606e4bcb

src/media/video/video_rtp_session.cpp

@@ -251,6 +251,7 @@ void VideoRtpSession::forceKeyFrame()
 void
 VideoRtpSession::setRotation(int rotation)
 {
+    if (receiveThread_)
         receiveThread_->setRotation(rotation);
 }