Returns the number of DATA payload in bytes received without WINDOW_UPDATE transmission for a connection. The local (receive) window size can be adjusted by
nghttp2_submit_window_update(). This function takes into account that and returns effective data length. In particular, if the local window size is reduced by submitting negative window_size_increment with
nghttp2_submit_window_update(), this function returns the number of bytes less than actually received.
This function returns -1 if it fails.