Don't crash on keyboard input with no handler

Prevent a call to put_kbd if null.

On shutdown of some OSes, the keyboard handler goes away before the
system is down. If a key is typed during this window, qemu crashes.

Signed-off-by: Don Koch <dkoch@verizon.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
