Fix a problem with reading uninitialized memory in ACM
Fix a problem with reading uninitialized memory in ACM

When an "empty frame" was produced by ACMGenericCodecWrapper::Encode,
the timestamp value was not set. This is now fixed, and the first byte
of the bitstream is set to something as well to avoid similar problems.

BUG=chromium:459483
R=jmarusic@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/34269004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8431}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 287755246a403a7a6e686bc921f59894669e362c

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc