The hnm demuxer's context struct contained lots of fields that are
write-only variables or that are not used outside of parsing the header
and that can therefore be replaced by local variables of hnm_read_header().
This commit removes all of these from the context; the second type has
been replaced by local variables.

An AVPacket (that was initialized when reading the header and for which
dead code to unreference it existed in hnm_read_close()) is among the
removed things. Removing it allowed to remove hnm_read_close()
altogether and also removes another instance of usage of sizeof(AVPacket).
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>