patch 9.2.0524: spell: buffer overflow with many affix or compound flags

patch 9.2.0524: spell: buffer overflow with many affix or compound flags

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: #20286

Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>