DefaultCallback on Login should process Erros first

[diogodamiani]

Hi,

I'm using the Facebook Login form, but when the user does not consent the app, it throws a NullReferenceException.

That is the CallStack:

Nemiro.OAuth.dll!Nemiro.OAuth.UniValue.IsNullOrEmpty(Nemiro.OAuth.UniValue value) Line 1136 C# Nemiro.OAuth.dll!Nemiro.OAuth.UniValue.operator !=(Nemiro.OAuth.UniValue x, string y) Line 2067 C# Nemiro.OAuth.LoginForms.dll!Nemiro.OAuth.LoginForms.Login.GetAccessToken(string authorizationCode) Line 400 C# Nemiro.OAuth.LoginForms.dll!Nemiro.OAuth.LoginForms.Login.AuthorizationCode.set(string value) Line 136 C# Nemiro.OAuth.LoginForms.dll!Nemiro.OAuth.LoginForms.Login.DefaultCallback(object sender, Nemiro.OAuth.LoginForms.WebBrowserCallbackEventArgs e) Line 351 C# Nemiro.OAuth.LoginForms.dll!Nemiro.OAuth.LoginForms.Login.webBrowser1_DocumentCompleted(object sender, System.Windows.Forms.WebBrowserDocumentCompletedEventArgs e) Line 274 C# Nemiro.OAuth.LoginForms.dll!Nemiro.OAuth.LoginForms.Login.webBrowser1_DocumentTitleChanged(object sender, System.EventArgs e) Line 236 C#

I think the erros in response should be treated first. Would you mind if I send a pull request?

Cheers