This is a backport of the fix for
MySQL BUG#52317: Assertion failing in Field_varstring::store () at field.cc:6833

The orginal comment by Oystein is:

In order for EXPLAIN to print const-refs, a Store_key_const_item object
is created. This is different for normal execution of subqueries where
a temporary store_key_item object is used instead. The problem is that
EXPLAIN will execute subqueries.  This leads to a scenario where a
store_key_const_item object it told to write to its underlying field.
This results in a failing assert since the write set of the underlying
table does not reflect this.  

The resolution is to do the same trick as for store_key_item::copy_inner().
That is, temporarily change the write set to allow writes to all columns.
This is only necessary in debug version since non-debug version does not
contain asserts on write_set.

sql/sql_select.h:
  Temporarily change write_set in store_key_const_item::copy_inner() to
  allow initialization of underlying field.  This is necessary since 
  subqueries are executed for EXPLAIN.  (For normal execution, 
  store_key_item::copy_inner is used.)