vtysh: fix potential stack buffer overflow

vtysh: fix potential stack buffer overflow

If vtysh was instructed to perform line-by-line processing on the output
of a command executed against a daemon and this output, as received by
vtysh, was not terminated with a newline, vtysh could print contents of
memory to its output device.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>