avformat/nutdec: fix packet end clearing

The code was buggy, using the wrong variable, also it missed the case
where the packet become smaller due to sidedata/metadata being extracted
which left a few bytes uninitialized

Fixes use of uninitialized memory
Fixed: msan_uninit-mem_7f6abbe44530_6838_mewmew_vorbis_ssa.nut
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
