EXPLAIN INSERT ... SELECT tried to use SELECT's execution path. This
caused a collection of problems:
- SELECT_DESCRIBE flag was not put into select_lex->options, which
  means it was not in JOIN::select_options either (except for the first
  member of the UNION).
- This caused UNION members to be executed. They would attempt to write
  join output rows to the output.
- (Actual cause of the crash) second join sibling would call
  result->send_eof() when finished execution. Then,
  Explain_query::print_explain would attempt to write to query output
  again, and cause an assertion due to non-empty query output.