avformat/nistspheredec: check sscanf() success before using the result

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f935c3c6c1a_7413_nist_pcms8.nist
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
