The problem was that LOAD DATA code (sql_load.cc) didn't take into
account that there may be items, representing references to other
columns. This is a usual case in views. The crash happened because
Item_direct_view_ref was casted to Item_user_var_as_out_param,
which is not a base class.

The fix is to
  1) Handle references properly;
  2) Ensure that an item is treated as a user variable only when
     it is a user variable indeed;
  3) Report an error if LOAD DATA is used to load data into
     non-updatable column.