Check for a null pointer after curl_easy_getinfo

[Martin Lemke]

I posted an issue and a patch on http://code.google.com/p/curlpp/issues/detail?id=18,
this is related to the fact that curl_easy_getinfo can return a null
pointer in some cases but this isn't handled well in the curlpp.

Please review this patch, if necessary I'd like to have it added it to
the curlpp repository.

Thanks!