Some SHOW statements call make_schema_query_block() to generate a
query towards an information schema table. This query then fetches
the data set that will be the result of the SHOW statement. A subset
of these SHOW statements allow an optional WHERE-clause. This applies
to SHOW OPEN TABLES, SHOW FIELDS/COLUMNS, and SHOW KEYS. If the
WHERE-clause contains a subquery (e.g. "SHOW OPEN TABLES WHERE 1 IN
(SELECT c1 FROM t1)"), the optimizer may try to transform the subquery
into a join between the information schema table and a derived table
(derived from "SELECT c1 FROM t1" in the example above). This triggers
an assert or segment violation, because Query_block.join_list at that
point is empty, i.e. there is nothing to join the derived table
with. Query_block.join_list is empty because make_schema_query_block()
fails to add the information schema table to join_list.

The fix alters make_schema_query_block() so that it adds the
information schema table to Query_block.join_list. It also adds
testcases to verify the fix.

Change-Id: I7a0ed7a133ffc739e0ef495446c9703d02e7b66a