The Uname name was never being used because it always generated a
too-long string.

The new test looking for zero bytes wouldn't have caught it (I thought
it would've), but is still nice to have.

Updates #24701

Change-Id: I2648074452609e4ad1b9736973e1b3a95eac658d
Reviewed-on: https://go-review.googlesource.com/110436Reviewed-by: Ian Lance Taylor <iant@golang.org>