index_read(), whereas it must return HA_ERR_END_OF_FILE
instead (as there are by definition no rows in a table of 
that engine.


mysql-test/r/blackhole.result:
  Bug#19717: test case
mysql-test/t/blackhole.test:
  Bug#19717: test case
sql/ha_blackhole.cc:
  Bug#19717: return no rows instead of success.