Fix unsynchronized overflow behavior to keep data available to read am: d0ace5eb35 am: f3d7571e32

Original change: https://android-review.googlesource.com/c/platform/system/libfmq/+/3119872

Change-Id: I81aad18c57ae37627126130f5cab2a8e54169f71
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>