fix crash on read returning -1

Reference: /n/atom/patch/applied/dirreadcrash
Date: Fri Mar 21 00:12:10 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libc/9sys/dirread.c	Fri Mar 21 00:12:10 2014
+++ /sys/src/libc/9sys/dirread.c	Fri Mar 21 00:12:10 2014
@@ -11,7 +11,7 @@
 
 	*d = nil;
 	if(ts <= 0)
-		return 0;
+		return ts;
 
 	/*
 	 * first find number of all stats, check they look like stats, & size all associated strings
@@ -62,8 +62,7 @@
 	if(buf == nil)
 		return -1;
 	ts = read(fd, buf, DIRMAX);
-	if(ts >= 0)
-		ts = dirpackage(buf, ts, d);
+	ts = dirpackage(buf, ts, d);
 	free(buf);
 	if(*d != nil)
 		setmalloctag(*d, getcallerpc(&fd));
@@ -90,8 +89,7 @@
 			break;
 		ts += n;
 	}
-	if(ts >= 0)
-		ts = dirpackage(buf, ts, d);
+	ts = dirpackage(buf, ts, d);
 	free(buf);
 	if(*d != nil)
 		setmalloctag(*d, getcallerpc(&fd));