Initialize payload-type frequency in channel.cc.

Uninitialized values triggered divide-by-zero crashes in voe_auto_test.

BUG=
R=stefan@webrtc.org, xians@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1780004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@4319 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/rtp_rtcp/source/rtp_receiver_impl.cc

@@ -334,6 +334,8 @@ bool RtpReceiverImpl::RetransmitOfOldPacket(const RTPHeader& header,
 
   CriticalSectionScoped cs(critical_section_rtp_receiver_);
   uint32_t frequency_khz = header.payload_type_frequency / 1000;
+  assert(frequency_khz > 0);
+
   int64_t time_diff_ms = clock_->TimeInMilliseconds() -
       last_receive_time_;