Don't crash the compiler when redirection sign is invalid.

This fixes #484.

eval/compile_op.go

@@ -378,6 +378,10 @@ func (cp *compiler) redir(n *parse.Redir) OpFunc {
 	sourceIsFd := n.RightIsFd
 	mode := n.Mode
 	flag := makeFlag(mode)
+	if flag == -1 {
+		// TODO: Record and get redirection sign position
+		cp.errorf("bad redirection sign")
+	}
 
 	return func(ec *EvalCtx) {
 		var dst int

eval/util.go

@@ -88,8 +88,7 @@ func makeFlag(m parse.RedirMode) int {
 	case parse.Append:
 		return os.O_WRONLY | os.O_CREATE | os.O_APPEND
 	default:
-		// XXX should report parser bug
-		panic("bad RedirMode; parser bug")
+		return -1
 	}
 }