Protect ACM decoder buffer in stereo.

In https://code.google.com/p/webrtc/source/detail?r=8730, I did a protection on ACM decoder buffer from being overflow.

However, the I misunderstood the return unit for PacketDuration(), and therefore, stereo decoders are not well protected.

This CL fixed this.

BUG=4361
R=henrik.lundin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/47289004

Cr-Commit-Position: refs/heads/master@{#9275}
4 files changed