DO NOT MERGE Use unsigned length when reading data

DO NOT MERGE Use unsigned length when reading data

With a signed length, invalid negative sizes can bypass data limit
checks of the type:

        if (data + length < end)

With an unsigned length, absurdly large lengths will now trigger an
early exit instead of following through into the decoding routine
with a bad length.

Bug: 5143832
Change-Id: I8e4a8d357ee04a36e35ab47d538ce57088734ccf