Repair crashes mysql when table has fulltext index.


myisam/sort.c:
  Use static ft_buf instead of dynamic mergebuf. Latter could be NULL if record has long words.
mysql-test/r/fulltext.result:
  Test case for BUG#11684 - repair crashes mysql when table has fulltext index.
mysql-test/t/fulltext.test:
  Test case for BUG#11684 - repair crashes mysql when table has fulltext index.