WHERE predicates containing references to empty tables in a
subquery were handled incorrectly by the optimizer when
executing EXPLAIN. As a result, the optimizer could try to
evaluate such predicates rather than just stop with
"Impossible WHERE noticed after reading const tables" as 
it would do in a non-subquery case. This led to valgrind 
errors and crashes.

Fixed the code checking the above condition so that subqueries
are not excluded and hence are handled in the same way as top
level SELECTs.

mysql-test/r/explain.result:
  Added a test case for bug #48419.
mysql-test/r/ps.result:
  Updated test results to take the new (and more correct)
  "Extra" comments in execution plans.
mysql-test/t/explain.test:
  Added a test case for bug #48419.
sql/sql_select.cc:
  There is no point in excluding subqueries from checking
  for identically false WHERE conditions.