Fixing a race condition in ACMGenericCodec

The old object was deleted before the pointer to it was removed from
the decoder proxy.

BUG=chromium:467209
TBR=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/49429004

Cr-Commit-Position: refs/heads/master@{#8736}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8736 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed