check return value of write

Reference: /n/atom/patch/applied2013/rxchkwrite
Date: Wed Jun 19 01:36:56 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/rx.c	Wed Jun 19 01:36:49 2013
+++ /sys/src/cmd/rx.c	Wed Jun 19 01:36:49 2013
@@ -106,7 +106,9 @@
 			return;
 		error("auth_proxy", nil);
 	}
-	write(fd, cmd, strlen(cmd)+1);
+	n = write(fd, cmd, strlen(cmd)+1);
+	if(n <= 0)
+		error("write error", 0);
 
 	kid = send(fd);
 	while((n=read(fd, buf, sizeof buf))>0)