tail(1): Do not print bogus errno string

In the case where write(2) does not return -1, it does not initialize errno.
This can happen when a broken pipe causes a short write.

I attempted to adapt the submitted test case to ATF but could not figure out
how to make the test run in the ATF environment. So the aborted test is
left disabled, in case someone would like to run it manually or fix it.

PR: 221976
Submitted by: <martin AT lispworks.com> (earlier version)
Sponsored by: Dell EMC Isilon