lib/vty: don't clear output buffer on input EOF

lib/vty: don't clear output buffer on input EOF

A VTY's input can be closed without the output becoming unavailable.
This happens both on stdio when stdin ends, as well as over TCP when an
unidirectional input shutdown() happens.

In such a case, resetting the output buffer is not appropriate since
there might still be data to be successfully written.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>