Fixes the problem with orphan fromnet() procs accumulating if their
respective fromstin() were hung up (read(0) < 0).

The SSH servers we have around (OpenSSH of many different versions) do
not cause hang up when they see SSH_CMSG_EOF alone.

Reference: /n/sources/patch/applied/ssh1-hangup
Date: Wed Dec 12 15:04:49 CET 2012
Signed-off-by: yarikos@gmail.com

--- /sys/src/cmd/ssh1/ssh1.c	Wed Dec 12 15:04:36 2012
+++ /sys/src/cmd/ssh1/ssh1.c	Wed Dec 12 15:04:33 2012
@@ -563,7 +563,8 @@
 		sendwritemsg(c, buf, n);
 	}
 	sendwritemsg(c, "", 0);
-	atexitdont(atexitkiller);
+	if(n >= 0)
+		atexitdont(atexitkiller);
 	exits(nil);
 }