Problem: JOIN::exec() tries to create temporary tables for derived
tables that the optimizer has decided to treat as constant tables.

Consider the query from the test case:

SELECT *
FROM (SELECT * FROM t1) AS a1
  RIGHT JOIN t3
  LEFT JOIN t2 ON d=c ON a=c;

The optimizer first decides to materialize a1. In
make_join_statistics() it calls update_ref_and_keys() to identify all
possible keys for the table. Later on, make_join_statistics()
discovers that the join conditions are impossible and tags a1 as a
constant table returning only NULL values. Unused keys are supposed to
be removed later on by JOIN::drop_unused_derived_keys(), but since a1
has been marked as a constant table, it is ignored and all the
possible keys are left.

At the start of query execution, JOIN::prepare_result() calls
st_select_lex::handle_derived() to create the necessary temporary
tables. When it tries to create a temporary table for a1, the attempt
triggers an assertion in create_myisam_tmp_table() since it tries to
create a temporary table with more than one key.

Fix: Check that a derived table is not marked as constant before
creating a temporary table for it.