netcat: avoid issuing syscalls on fd -1

netcat: avoid issuing syscalls on fd -1

In case a socket error condition occurs, readwrite() invalidates the
corresponding fd. Later on, readwrite() may still issue a syscall on
it. Avoid that by adding a couple of checks for fd == -1.

Reported and fix suggested by Leah Neukirchen.
Fixes https://github.com/libressl/openbsd/issues/143

"looks right" deraadt