Fix a bug in ACM test channel
Fix a bug in ACM test channel

The test code could read outside the allocated memory. The bug could up
until now not be triggered by the production code, but coming changes
would uncover it.

COAUTHOR=kwiberg@webrtc.org
R=minyue@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/34929004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8216}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 05db352f56b369444e1a83c5c85a4fb513bf60f0

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