The problem affected queries in form:

  SELECT FROM (SELECT where Split Materialized is applicable) WHERE 1=0

The problem was caused by this:
- The select in derived table uses two-phase optimization (due to a
  possible Split Materialized).
- The primary select has "Impossible where" and so it short-cuts its
  optimization.
- The optimization for the SELECT in the derived table is never finished,
  and EXPLAIN data structure has a dangling pointer to select #2.

Fixed with this: make JOIN::optimize_stage2() invoke optimization of
derived tables when it is handing a degenerate JOIN with zero tables.
We will not execute the derived tables but we need their query plans
for [SHOW]EXPLAIN.