fix crash caused by non-nil response_data but nil token/secret in respon...

...se

It's possible that response_data is NOT nil, while the response is nil.
We have observed many real crashes due to following code assigning nil value to a dict:
    fullResponse[@"oauth_token_secret"] = response[@"oauth_token_secret"];
    fullResponse[@"oauth_token"]        = response[@"oauth_token"];