MDEV-9986 Full-text search of the utf8mb4 column causes crash

take into account that agg_arg_charsets_for_comparison()
can replace Item_field's with Item_func_conv_charset

mysql-test/r/fulltext3.result

@@ -15,3 +15,15 @@ CREATE TABLE t1(a VARCHAR(2) CHARACTER SET big5 COLLATE big5_chinese_ci,
 FULLTEXT(a));
 INSERT INTO t1 VALUES(0xA3C2);
 DROP TABLE t1;
+create table t1 (
+id varchar(255),
+business_name text null collate utf8mb4_unicode_ci,
+street_address text,
+fulltext index ft (business_name),
+fulltext index ft2 (street_address)
+);
+select * from t1 where match (business_name, street_address) against ('some business name and address here');
+ERROR HY000: Can't find FULLTEXT index matching the column list
+select * from t1 where match (business_name, street_address) against ('some business name and address here' in boolean mode);
+id	business_name	street_address
+drop table t1;

mysql-test/t/fulltext3.test

@@ -32,3 +32,18 @@ INSERT INTO t1 VALUES(0xA3C2);
 DROP TABLE t1;
 
 # End of 5.1 tests
+
+#
+# MDEV-9986 Full-text search of the utf8mb4 column causes crash
+#
+create table t1 (
+  id varchar(255),
+  business_name text null collate utf8mb4_unicode_ci,
+  street_address text,
+  fulltext index ft (business_name),
+  fulltext index ft2 (street_address)
+);
+--error ER_FT_MATCHING_KEY_NOT_FOUND
+select * from t1 where match (business_name, street_address) against ('some business name and address here');
+select * from t1 where match (business_name, street_address) against ('some business name and address here' in boolean mode);
+drop table t1;

sql/item_func.cc

@@ -6366,6 +6366,8 @@ bool Item_func_match::fix_index()
 
   for (i=1; i < arg_count; i++)
   {
+    if (args[i]->type() != FIELD_ITEM)
+      goto err;
     item=(Item_field*)args[i];
     for (keynr=0 ; keynr < fts ; keynr++)
     {