The Item_func_set_user_var::register_field_in_read_map() did not check 
that the result_field was null.This caused server crashes for queries that
required order by such a field and were executed without using a temporary
table.

The Item_func_set_user_var::register_field_in_read_map() now checks the
result_field to be not null.


mysql-test/t/user_var.test:
  Added a test case for the bug#32482: Crash for a query with ORDER BY a user variable.
mysql-test/r/user_var.result:
  Added a test case for the bug#32482: Crash for a query with ORDER BY a user variable.
sql/item_func.cc:
  Bug#32482: Crash for a query with ORDER BY a user variable.
  The Item_func_set_user_var::register_field_in_read_map() now checks the
  result_field to be not null.