Analysis:
  The failed assert ensured that the choice of subquery strategy
  is performed only for queries with at least one table. If there
  is a LIMIT 0 clause all tables are removed, and the subquery is
  neither optimized, nor executed during actual optimization. However,
  if the query is EXPLAIN-ed, the EXPLAIN execution path doesn't remove
  the query tables if there is a LIMIT 0 clause. As a result, the
  subquery optimization code is called, which violates the ASSERT
  condition.
  
Solution:
  Transform the assert into a condition, and if the outer query
  has no tables assume that there will be at most one subquery
  execution.
  
  There is potentially a better solution by reengineering the
  EXPLAIN/optimize code, so that subquery optimization is not
  done if not needed. Such a solution would be a lot bigger and
  more complex than a bug fix.