Problem: spell: a word in a .dic file with many postponed prefix or
compound flags overflows the fixed-size store_afflist[MAXWLEN]
buffer in get_pfxlist() and get_compflags().
Solution: Add bounds checks (Yasuhiro Matsumoto).
closes: vim/vim#202869a920e8254
Co-authored-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>