AudioBuffer: Let ChannelBuffer handle bounds checking of channel parameter

R=aluebs@webrtc.org, minyue@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/13019004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6714 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_processing/common.h

@@ -55,7 +55,7 @@ class ChannelBuffer {
 
   T* data() { return data_.get(); }
   T* channel(int i) {
-    assert(i < num_channels_);
+    assert(i >= 0 && i < num_channels_);
     return channels_[i];
   }
   T** channels() { return channels_.get(); }