[automerger skipped] Add missing signedness check in Keymaster buffer am: 7f7cabaaee am: c53149e27c -s ours

am skip reason: Merged-In I15330a2f23c3461e23daad450af33e3f92e6730c with SHA-1 48edbcdb98 is already in history

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/keymaster/+/15836250

Change-Id: I6a7e7c24e783533c84019216914ad10f263047b3