[scudo] Don't unmap other low memory pages if mapping doesn't exist am: 964d4f2c92

Original change: https://android-review.googlesource.com/c/platform/external/scudo/+/2045286

Change-Id: I1e6eabf112d03de7eb7eb6a45f3d457daa819139
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>