Fixes http://code.google.com/p/webrtc/issues/detail?id=941

BUG=941

Review URL: https://webrtc-codereview.appspot.com/966020

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@3125 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/voice_engine/channel.cc b/voice_engine/channel.cc
index 976146f..015518c 100644
--- a/voice_engine/channel.cc
+++ b/voice_engine/channel.cc
@@ -332,7 +332,10 @@
     // Packet transmission using external transport transport
     {
         CriticalSectionScoped cs(&_callbackCritSect);
-
+        if (_transportPtr == NULL)
+        {
+            return -1;
+        }
         int n = _transportPtr->SendRTCPPacket(channel,
                                               bufferToSendPtr,
                                               bufferLength);