fulltext index

Having a table with broken multibyte characters may cause fulltext
parser dead-loop.

Since normally it is not possible to insert broken multibyte sequence
into a table, this problem may arise only if table is damaged.

Affected statements are:
- CHECK/REPAIR against damaged table with fulltext index;
- boolean mode phrase search against damaged table with or
  without fulltext inex;
- boolean mode searches without index;
- nlq searches.

No test case for this fix. Affects 5.0 only.