box/lua/console: Fix hang in case of cancelling console fiber

Since commit f42596a2 we've started
to test if a fiber is cancelled in coio_wait. This may hang interactive
console with simple fiber.kill(fiber.self()) call.

Sane users don't do such tricks and this affects interactive console
mode only but still to be on a safe side lets exit if someone occasionally
killed the console fiber.

Notes:
 - such exit may ruine terminals settings and one need
   to reset it after;
 - the issue happens on interactive console only
   so I didn't find a way for automatic test and
   tested manually.