disabled if ref is built with a key from the updated table

Problem was in add_not_null_conds() optimization function.
It contains following code:
JOIN_TAB *referred_tab= not_null_item->field->table->reginfo.join_tab;
...
add_cond_and_fix(&referred_tab->select_cond, notnull);
For UPDATE described in bug report referred_tab is 0 and dereferencing it
crashes the server.



sql/sql_select.cc:
  Fix bug #11868 NOT NULL ref optimization in subquery used in update must be
  disabled if ref is built with a key from the updated table
mysql-test/r/update.result:
  Test case for bug#11868 Update with subquery with ref built with a key from
  the updated table crashes server.
mysql-test/t/update.test:
  Test case for bug#11868 Update with subquery with ref built with a key from the updated table crashes server