avcodec/kgv1dec: fix memleak, reintroduce lost decode_flush() call

fixes regression since 80d44190c4d909e9685d04bcde693da3e2441dfd and 1a0370ad94de094a5ac754e790d4ec8d8d0d9ee7
this reverts the buggy changes in these 2 commits

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>