When the input is /dev/null, every read returns EOF. In general, it
may also happen that read doesn't work as intended. So, the robust thing
to do here is to ensure that we check the return values. By doing that
we notice of io.EOF errors and we don't proceed with the onboarding.
This diff fixes the issue described by https://github.com/ooni/probe/issues/1281
however it may be that we also want (in the near or not-so-near future)
to stop onboarding if the input terminal is not a tty. This is however a
possible future evolution that should not prevent us for committing and
merging this simple fix that unblocks creating a Debian package.
e4ef279b80