Fix comparison-always-true warning with -Wextra.

TEST=build on Linux with -Wextra.

Review URL: https://webrtc-codereview.appspot.com/353005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@1456 4adac7df-926f-26a2-2b94-8c16560cd09d

src/modules/rtp_rtcp/source/transmission_bucket.cc

@@ -108,7 +108,7 @@ int32_t TransmissionBucket::GetNextPacket() {
   bytes_rem_total_ -= num_bytes;
   bytes_rem_interval_ -= num_bytes;
 
-  assert(accumulator_ - num_bytes >= 0);
+  assert(accumulator_ >= num_bytes);
   accumulator_ -= num_bytes;
 
   packets_.erase(packets_.begin());