net/dns: detect and decode UTF-16 from wsl.exe earlier.

Fixes #3170

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/utf_test.go

@@ -0,0 +1,25 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dns
+
+import "testing"
+
+func TestMaybeUnUTF16(t *testing.T) {
+	tests := []struct {
+		in   string
+		want string
+	}{
+		{"abc", "abc"},             // UTF-8
+		{"a\x00b\x00c\x00", "abc"}, // UTF-16-LE
+		{"\x00a\x00b\x00c", "abc"}, // UTF-16-BE
+	}
+
+	for _, test := range tests {
+		got := string(maybeUnUTF16([]byte(test.in)))
+		if got != test.want {
+			t.Errorf("maybeUnUTF16(%q) = %q, want %q", test.in, got, test.want)
+		}
+	}
+}