Fixed the overflow in the AGC

BUG=webrtc:8236

Review-Url: https://codereview.webrtc.org/3009373002
Cr-Original-Commit-Position: refs/heads/master@{#19818}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: fb2fa3f54e97c4465a6fd6fd992903a52397d739