Unmarshals the header and returns the length of the payload
@param evbuf the buffer from which to unmarshal data @param ptag a pointer in which the tag id is being stored @returns -1 on failure or the number of bytes in the remaining payload.
See Implementation
Unmarshals the header and returns the length of the payload
@param evbuf the buffer from which to unmarshal data @param ptag a pointer in which the tag id is being stored @returns -1 on failure or the number of bytes in the remaining payload.