In Item_cache_decimal::store(item) the call item->val_decimal_result() 
returns NULL if the passed item has an SQL null value. Don't try copying
NULL into Item_cache_decimal::val in this case.


mysql-test/r/type_decimal.result:
  Test for BUG#8397
mysql-test/t/type_decimal.test:
  Test for BUG#8397
sql/item.cc:
  Fix for BUG#8397: In Item_cache_decimal::store(item) the call 
  item->val_decimal_result() returns NULL if the passed item has an SQL null 
  value. Don't try copying NULL into Item_cache_decimal::val in this case.