avformat/nistspheredec: initialize header_size to -1

Fixes use of uninitialized memory
This also avoids the need to check the scanf() return as the subsequent check catches it
Fixes: msan_uninit-mem_7f712e4dc173_7414_nist_ulaw.nist
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>