libbinder: Use correct data size in appendFrom test

I noticed this because I had temporarily changed the order of the checks
for a WIP CL and this started failed in the wrong way.

There is a fuzzer for appendFrom, so this shouldn't be a loss of
coverage for the bad data size case.

Test: binderRpcTest
Change-Id: Ia86681c59b2f814f93298d2837d8dcb03c84bdb0