If an outer query is broken, a subquery might not even get set up.
EXPLAIN EXTENDED did not expect this and merrily tried to de-ref all
of the half-setup info.

We now catch this case and print as much as we have, as it doesn't cost us
anything (doesn't make regular execution slower).

mysql-test/r/explain.result:
  Show that EXPLAIN EXTENDED with subquery and illegal out query doesn't crash.
  Show also that SHOW WARNINGS will render an additional Note in the hope of
  being, well, helpful.
mysql-test/t/explain.test:
  If we have only half a query for EXPLAIN EXTENDED to print (i.e.,
  incomplete subquery info as outer query is illegal), we should
  provide the user with as much info as we easily can if they ask
  for it. What we should not do is crash when they come asking for
  help, that violates etiquette in some countries.
sql/item_subselect.cc:
  If the sub-query's actually set up, print it. Otherwise, elide.