Remove unused param to asf_parse_headerext.
Remove unused param to asf_parse_headerext.

The buflen parameter passed to the function is a part of
the asf_object_headerext_t argument, and was never used
locally, and the datalen member of asf_object_headerext_t
is even overwritten while parsing the header extension.

juhovh/libasf,dsvensson/libasf,juhovh/libasf