Not only the first, but each latest chunk must be cached to allow
seekback after finding the mime boundary.

Fixes trac #5023 and #5921.
Signed-off-by: Moritz Barsnick <barsnick@gmx.net>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>