BUG#31950 - repair table hangs while processing multicolumn utf8

            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.

myisam/ft_parser.c

@@ -188,7 +188,7 @@ byte ft_simple_get_word(CHARSET_INFO *cs, byte **start, const byte *end,
 
   do
   {
-    for (;; doc+= mbl)
+    for (;; doc+= (mbl ? mbl : 1))
     {
       if (doc >= end) DBUG_RETURN(0);
       if (true_word_char(cs, *doc)) break;