The cause of the bug was an ASSERT that checked the consistency
of TABLE_SHARE::db and TABLE_LIST::db and failed for I_S tables.
The fix relaxes the requirement for consistency for I_S.


mysql-test/r/join.result:
  Added test for BUG#17523
mysql-test/t/join.test:
  Added test for BUG#17523
sql/table.cc:
  Take into account that for I_S tables, TABLE_SHARE::db == 0,
  while TABLE_LIST::db contains the database name of a table.
  The only change is in the ASSERTs.