create_sort_index() function overwrites original JOIN_TAB::type field.
At re-execution of subquery overwritten JOIN_TAB::type(JT_ALL) is
used instead of JT_FT. It misleads test_if_skip_sort_order() and
the function tries to find suitable key for the order that should
not be allowed for FULLTEXT(JT_FT) table.
The fix is to restore JOIN_TAB strucures for subselect on re-execution
for EXPLAIN.
Additional fix:
Update TABLE::maybe_null field which
affects list_contains_unique_index() behaviour as it
could have the value(maybe_null==TRUE) based on the
assumption that this join is outer
(see setup_table_map() func).


mysql-test/r/explain.result:
  test case
mysql-test/t/explain.test:
  test case
sql/item_subselect.cc:
  Make subquery uncacheable in case of EXPLAIN. It allows to keep
  original JOIN_TAB::type(see JOIN::save_join_tab) and restore it
  on re-execution.
sql/sql_select.cc:
  -restore JOIN_TAB strucures for subselect on re-execution for EXPLAIN
  -Update TABLE::maybe_null field as it could have
   the value(maybe_null==TRUE) based on the assumption
   that this join is outer(see setup_table_map() func).
   This change is not related to the crash problem but
   affects EXPLAIN results in the test case.