Analysis:
This another instance of the problem fixed in LP BUG#675981 -
evaluation of subqueries during EXPLAIN when the subquery plan
is incomplete because JOIN::optimize() generally doesn't create
complete execution plans for EXPLAIN statements.

In this case the call path is:
mysql_explain_union -> outer_join.exec -> outer_join.init_execution ->
create_sort_index -> filesort -> find_all_keys ->
SQL_SELECT::skip_record -> outer_where_clause.val_int -> ...
-> subselect_join.exec -> ... -> sub_select_cache

When calling sub_select_cache JOIN_TAB::cache is NULL because the cache
objects are not created for EXPLAIN statements.

Solution:
Delay the call to init_execution() after all EXPLAIN related processing
is completed. Thus init_execution() is not called at all during EXPLAIN.