Prevent integer overflow in copyString am: 72de6c81aa am: 3d73d4e17a am: 983a4dcc60

Original change: https://googleplex-android-review.googlesource.com/c/platform/external/expat/+/18587368

Change-Id: Ic7b70abd52e6aafa08f706db1c8e955035b3acd7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>