make sure that we don't attempt to read from /dev/null on chat consoles

Reference: /n/sources/patch/applied/consoles
Date: Mon Jun 12 22:37:42 CES 2006
Signed-off-by: andrey@lanl.gov

--- /sys/src/cmd/aux/consolefs.c	Mon Jun 12 22:37:15 2006
+++ /sys/src/cmd/aux/consolefs.c	Mon Jun 12 22:37:12 2006
@@ -587,6 +587,8 @@
 	c = a[1];
 	c->pid = getpid();
 	notify(handler);
+	if(c->chat)
+		threadexits(nil);
 	for(;;){
 		n = read(c->fd, buf, sizeof(buf));
 		if(n < 0)