avcodec/eatgv: use av_mallocz() for frame_buffer

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7fbf6159587b_2571_ORIGIN.TGV
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
