Fixed ASAN failure for the test main.func_misc.

For Item name_const , we should never do typecast it to Item_field because we
always expect it to be a constant value.
So instead of checking the type() its better to introduce a function in the
Item class get_item_field, which would return the item_field object for the item
which have type of FIELD_ITEM

sql/item.h

@@ -881,6 +881,7 @@ class Item: public Value_source,
   */
   String *val_str() { return val_str(&str_value); }
   virtual Item_func *get_item_func() { return NULL; }
+  virtual Item_field *get_item_field() {return NULL;}
 
   const MY_LOCALE *locale_from_val_str();
 
@@ -3261,6 +3262,7 @@ class Item_field :public Item_ident,
   longlong val_int_endpoint(bool left_endp, bool *incl_endp);
   bool get_date(MYSQL_TIME *ltime, ulonglong fuzzydate);
   bool get_date_result(MYSQL_TIME *ltime,ulonglong fuzzydate);
+  Item_field* get_item_field() {return this;}
   bool is_null() { return field->is_null(); }
   void update_null_value();
   void update_table_bitmaps()

sql/sql_yacc.yy

@@ -292,13 +292,14 @@ LEX::set_system_variable(enum enum_var_type var_type,
                          Item *val)
 {
   set_var *setvar;
+  Item_field *item_field;
 
   /* No AUTOCOMMIT from a stored function or trigger. */
   if (spcont && sysvar == Sys_autocommit_ptr)
     sphead->m_flags|= sp_head::HAS_SET_AUTOCOMMIT_STMT;
 
-  if (val && val->type() == Item::FIELD_ITEM &&
-      ((Item_field*)val)->table_name)
+  if (val && (item_field= val->get_item_field()) &&
+      item_field->table_name)
   {
     my_error(ER_WRONG_TYPE_FOR_VAR, MYF(0), sysvar->name.str);
     return TRUE;