well, oops. on slow io (as can happen with ssh remote builders) we could
have extended a nar read buffer past what was actually read, injecting a
span of zeroes into the read buffer where we requested some data but got
a partial result instead. also add some tests that would've caught this.
Change-Id: I67aa06b4715aeec6a5bdacaafa9b79849e664e2f
eldritch horrors
93c3ca4e92