avformat/wc3movie: Check strings before printing.

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f7812ca062f_2812_SC_32_part.MVE
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
