MDEV-11241 Certain combining marks cause MariaDB to crash when doing Full-Text searches

Don't assume that a word of n bytes can match a word of
at most n * charset->mbmaxlen bytes, always go for the worst.

mysql-test/r/fulltext_charsets.result

+set names utf8mb4;
+create table t1 (a int, b text, fulltext (b)) charset=utf8mb4 collate=utf8mb4_unicode_ci;
+insert t1 values (1000, 'C͓̙̯͔̩ͅͅi̩̘̜̲a̯̲̬̳̜̖̤o͕͓̜͓̺̖̗,̠̬͚ ̺T͇̲h͈̱e ̬̜D̖o̦̖͔̗͖̩̘c̣̼t̝͉̫̮̗o͉̫̭r̙͎̗.͓̪̥');
+select a from t1 where match(b) against ('ciao' in boolean mode);
+a
+1000
+drop table t1;

mysql-test/t/fulltext_charsets.test

+#
+# MDEV-11241 Certain combining marks cause MariaDB to crash when doing Full-Text searches
+#
+set names utf8mb4;
+
+create table t1 (a int, b text, fulltext (b)) charset=utf8mb4 collate=utf8mb4_unicode_ci;
+insert t1 values (1000, 'C͓̙̯͔̩ͅͅi̩̘̜̲a̯̲̬̳̜̖̤o͕͓̜͓̺̖̗,̠̬͚ ̺T͇̲h͈̱e ̬̜D̖o̦̖͔̗͖̩̘c̣̼t̝͉̫̮̗o͉̫̭r̙͎̗.͓̪̥');
+select a from t1 where match(b) against ('ciao' in boolean mode);
+drop table t1;
+

storage/myisam/ft_boolean_search.c

@@ -195,12 +195,7 @@ static int ftb_query_add_word(MYSQL_FTPARSER_PARAM *param,
   switch (info->type) {
     case FT_TOKEN_WORD:
       ftbw= (FTB_WORD *)alloc_root(&ftb_param->ftb->mem_root,
-                                   sizeof(FTB_WORD) +
-                                   (info->trunc ? HA_MAX_KEY_BUFF :
-                                    (word_len + 1) *
-                                    ftb_param->ftb->charset->mbmaxlen +
-                                    HA_FT_WLEN +
-                                    ftb_param->ftb->info->s->rec_reflength));
+                                   sizeof(FTB_WORD) + HA_MAX_KEY_BUFF);
       ftbw->len= word_len + 1;
       ftbw->flags= 0;
       ftbw->off= 0;