Ensure that the ring buffer does not return a pointer into the buffer if
no data is available to read.
The ring buffer fix is not directly applicable to issue webrtc:7845, but may cause related memory errors.
BUG=webrtc:7845
Review-Url: https://codereview.webrtc.org/2971313002
Cr-Commit-Position: refs/heads/master@{#18940}