curl callbacks reading from `rest_internal::WriteVector` when no more data is present

https://github.com/googleapis/google-cloud-cpp/blob/36c3aeef8fdfb894e93cb49ce2fd79e0c7e987f2/google/cloud/internal/curl_writev.cc#L36 leads to a crash when no data is left