avoid extra blank lines

Reference: /n/atom/patch/applied2013/fmtupd
Date: Wed Jun 19 03:43:30 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/fmt.c	Wed Jun 19 03:43:23 2013
+++ /sys/src/cmd/fmt.c	Wed Jun 19 03:43:23 2013
@@ -162,6 +162,8 @@
 	Word *line;
 
 	/* one output line per loop */
+	if(nw == 0)
+		return;
 	line = *w;
 	fold = 0;
 	for(i=0; i<nw; ){