hangs 100% cpu

Fulltext parser may fall into infinite loop when it gets illegal
multibyte sequence (or a sequence that doesn't have mapping to unicode).

Affects 5.1 only.


mysql-test/r/fulltext.result:
  A test case for BUG#29464.
mysql-test/t/fulltext.test:
  A test case for BUG#29464.
storage/myisam/ft_parser.c:
  ctype() may return negative value, which was stored in unsigned
  variable.
  
  Also ctype() may return negative length for correct multibyte
  sequence that doesn't have a mapping to unicode. These characters
  are skipped correctly with this patch.