avformat/4xm: shrink packet if it was only partially initialized

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f6c76ab7950_4920_dracula.4xm
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
