cmListFileLexer: Fix leak on error before EOF

Teach cmListFileLexerDestroy to call cmListFileLexerSetToken with a NULL
token to free the token string buffer.  Without this, if an error occurs
before the token cleanup happens when EOF is reached, then the token
string buffer may leak.
Merge topic 'cmListFileLexer-fix-leak-on-error' into next

ad3d5c5 cmListFileLexer: Fix leak on error before EOF