libbinder: check null bytes in readString*Inplace

This is entirely defensive, since the only real guarantee we have here
from these APIs is that a buffer of a given length is available.
However, since we write 0's here, presumably to guard against people
assuming these are null-terminated strings, we might as well enforce
that they are actually null terminated.

Bug: 172655291
Test: binderParcelTest (added in newer CL)
Change-Id: Ie879112540155f6a93b97aeaf3d41ed8ba4ae79f
Merged-In: Ie879112540155f6a93b97aeaf3d41ed8ba4ae79f
(cherry picked from commit 51e02b16)