fix out-of-bounds read in normalize_cond_str on unterminated class

[arshsmith1]

normalize_cond_str scans the members of a [...] character class looking for the closing ], but the inner loop never checks for the string terminator. When an affix file opens a class and never closes it (say a condition field of [ab), that scan runs off the end of the heap buffer holding the field, which is an out-of-bounds read and a one-byte out-of-bounds write from the min swap a few lines down. The added terminator check reports the unterminated class through the existing invalid_cond path and leaves well-formed conditions alone.

Minimal repro with a two-line affix file (xx_affix.dat):

SET iso8859-1
SFX A Y 1
SFX A 0 s [ab

plus a matching xx.dat containing affix xx, then aspell --lang=xx --data-dir=. --local-data-dir=. --dict-dir=. create master ./xx.rws. An ASan build reports a heap-buffer-overflow in normalize_cond_str at the class scan (affix.cpp:224), reached through AffixMgr::parse_file. With the patch aspell instead prints The condition "[ab" is invalid.