Return 0 when read/write ptrs are incorrect am: 57c8b9df52

Original change: https://android-review.googlesource.com/c/platform/system/libfmq/+/2918987

Change-Id: I2b15da5f106ab9e05cdca3a81e3b40269ee04cb0
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>