[scudo] Slightly improve the handling of last block in a region am: f2b97c0187

Original change: https://android-review.googlesource.com/c/platform/external/scudo/+/2480075

Change-Id: Ifc53bc85d391875611d15747fe4018f2336629a1
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>