MDEV-15738 Server crashes in my_strcasecmp_utf8 on query from I_S with UNION executed as PS

Problem was the Item_field::Item_field(THD*, Field*) had old code
that put a null pointer in orig_field_names. Now, when we have
proper re-prepare if table definition changes, this is not needed
anymore.

mysql-test/main/prepare.result

+PREPARE stmt1 FROM "
+SELECT table_name FROM information_schema.tables
+WHERE table_name = 't1_first'
+UNION ALL
+SELECT table_name FROM information_schema.tables
+WHERE table_name = 't1_second'";
+execute stmt1;
+table_name
+execute stmt1;
+table_name
+create or replace table t1 (a int primary key, table_name char(40));
+insert into t1 values(1,"t1_first");
+insert into t1 values(2,"t1_second");
+PREPARE stmt2 FROM "
+SELECT table_name FROM t1
+WHERE table_name = 't1_first'
+UNION ALL
+SELECT table_name FROM t1
+WHERE table_name = 't1_second'";
+execute stmt2;
+table_name
+t1_first
+t1_second
+execute stmt2;
+table_name
+t1_first
+t1_second
+flush tables;
+execute stmt2;
+table_name
+t1_first
+t1_second
+alter table t1 add column b int;
+execute stmt2;
+table_name
+t1_first
+t1_second
+execute stmt2;
+table_name
+t1_first
+t1_second
+drop table t1;
+execute stmt2;
+ERROR 42S02: Table 'test.t1' doesn't exist
+create or replace table t1 (a int primary key, table_name char(40));
+insert into t1 values(1,"t1_first");
+execute stmt2;
+table_name
+t1_first
+deallocate prepare stmt1;
+deallocate prepare stmt2;
+drop table t1;

mysql-test/main/prepare.test

+#
+# MDEV-15738 Server crashes in my_strcasecmp_utf8 on query from I_S with UNION
+# executed as PS
+#
+
+PREPARE stmt1 FROM "
+SELECT table_name FROM information_schema.tables
+WHERE table_name = 't1_first'
+UNION ALL
+SELECT table_name FROM information_schema.tables
+WHERE table_name = 't1_second'";
+execute stmt1;
+execute stmt1;
+
+create or replace table t1 (a int primary key, table_name char(40));
+insert into t1 values(1,"t1_first");
+insert into t1 values(2,"t1_second");
+
+PREPARE stmt2 FROM "
+SELECT table_name FROM t1
+WHERE table_name = 't1_first'
+UNION ALL
+SELECT table_name FROM t1
+WHERE table_name = 't1_second'";
+
+execute stmt2;
+execute stmt2;
+flush tables;
+execute stmt2;
+alter table t1 add column b int;
+execute stmt2;
+execute stmt2;
+drop table t1;
+--error ER_NO_SUCH_TABLE
+execute stmt2;
+create or replace table t1 (a int primary key, table_name char(40));
+insert into t1 values(1,"t1_first");
+execute stmt2;
+
+deallocate prepare stmt1;
+deallocate prepare stmt2;
+drop table t1;

sql/item.cc

@@ -3077,12 +3077,9 @@ Item_field::Item_field(THD *thd, Field *f)
    have_privileges(0), any_privileges(0)
 {
   set_field(f);
-  /*
-    field_name and table_name should not point to garbage
-    if this item is to be reused
-  */
-  orig_table_name= "";
-  orig_field_name= null_clex_str;
+
+  orig_table_name= table_name;
+  orig_field_name= field_name;
   with_field= 1;
 }