Bug#19599 duplication of information_schema column value in a CONCAT expr with user var

mark result string using String::mark_as_const()
which prevents CONCAT from reusing it as a buffer
for concatenation result.

mysql-test/r/information_schema.result

@@ -1121,3 +1121,12 @@ NULL	test	v2	select 1 AS `1`	NONE	NO	mysqltest_1@localhost	DEFINER
 drop view v1, v2;
 drop table t1;
 drop user mysqltest_1@localhost;
+set @a:= '.';
+create table t1(f1 char(5));
+create table t2(f1 char(5));
+select concat(@a, table_name), @a, table_name
+from information_schema.tables where table_schema = 'test';
+concat(@a, table_name)	@a	table_name
+.t1	.	t1
+.t2	.	t2
+drop table t1,t2;

mysql-test/t/information_schema.test

@@ -838,3 +838,13 @@ connection default;
 drop view v1, v2;
 drop table t1;
 drop user mysqltest_1@localhost;
+
+#
+# Bug#19599 duplication of information_schema column value in a CONCAT expr with user var
+#
+set @a:= '.';
+create table t1(f1 char(5));
+create table t2(f1 char(5));
+select concat(@a, table_name), @a, table_name
+from information_schema.tables where table_schema = 'test';
+drop table t1,t2;

sql/item_strfunc.h

@@ -674,6 +674,7 @@ class Item_func_conv_charset :public Item_str_func
                                  str->charset(), conv_charset, &errors))
         null_value= 1;
       use_cached_value= 1;
+      str_value.mark_as_const();
       safe= (errors == 0);
     }
     else