VCM/Receiver: Return null when can't extract frame.

R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1435004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@3974 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/webrtc/modules/video_coding/main/source/receiver.cc b/webrtc/modules/video_coding/main/source/receiver.cc
index 3457260..38c5db6 100644
--- a/webrtc/modules/video_coding/main/source/receiver.cc
+++ b/webrtc/modules/video_coding/main/source/receiver.cc
@@ -226,7 +226,9 @@
 
   // Extract the frame from the jitter buffer and set the render time.
   VCMEncodedFrame* frame = jitter_buffer_.ExtractAndSetDecode(frame_timestamp);
-  assert(frame);
+  if (frame == NULL) {
+    return NULL;
+  }
   frame->SetRenderTime(next_render_time_ms);
   if (dual_receiver != NULL) {
     dual_receiver->UpdateState(*frame);