The Cached_item_decimal::cmp() method wasn't checking for null pointer
returned from the val_decimal() of the item being cached.
This leads to server crash.

The Cached_item_decimal::cmp() method now check for null values.


sql/item_buff.cc:
  Bug#22138: Unhandled NULL caused server crash
  The Cached_item_decimal::cmp() method now check for null values.
mysql-test/r/type_decimal.result:
  Added the test case for bug#22138: Unhandled NULL caused server crash
mysql-test/t/type_decimal.test:
  Added the test case for bug#22138: Unhandled NULL caused server crash