Bugfix when no input file specified

This reverts commit 74ad307bd1.

main.go

@@ -33,7 +33,7 @@ import (
 )
 
 const (
-	version = "0.5.9c"
+	version = "0.6.1"
 )
 
 var (
@@ -84,6 +84,10 @@ func main() {
 	}
 
 	args := os.Args
+	if len(os.Args) < 2 {
+		pflag.Usage()
+		os.Exit(1)
+	}
 	filename := args[len(args)-1]
 
 	var f *os.File