The problem was with Materialized_cursor and temporary table it uses.
Temorary table's fields had Field::orig_table pointing to the tables
that were used in the query that produced data for the cursor.
When "FETCH INTO sp_var" statement is executed, those original tables
were already closed.  However, copying from Materialized_cursor's table
into SP variable may cause field_conv() to be invoked which calls
field->type() which may access field->orig_table (for certain field types).

Fixed by setting Materialized_cursor->table->field[i]->orig_table to point
to Materialized_cursor->table.  (this is how it is done for regular base
tables)