http: avoid crash when asked for multiple file ranges

R=adg
CC=golang-dev
https://golang.org/cl/4289076

src/pkg/http/fs.go

@@ -154,7 +154,10 @@ func serveFile(w ResponseWriter, r *Request, name string, redirect bool) {
 	// handle Content-Range header.
 	// TODO(adg): handle multiple ranges
 	ranges, err := parseRange(r.Header.Get("Range"), size)
-	if err != nil || len(ranges) > 1 {
+	if err == nil && len(ranges) > 1 {
+		err = os.ErrorString("multiple ranges not supported")
+	}
+	if err != nil {
 		Error(w, err.String(), StatusRequestedRangeNotSatisfiable)
 		return
 	}