get_best_covering_ror_intersect() was copying ror_scans starting from the 
end of the array and not from its beginning.


mysql-test/r/index_merge_innodb.result:
  Testcase for BUG#19021
mysql-test/t/index_merge_innodb.test:
  Testcase for BUG#19021
sql/opt_range.cc:
  BUG#19021: In get_best_covering_ror_intersect(), the array of pointers to
  scans to be used is [tree->ror_scans, ror_scan_mark), and not
  [ror_scan_mark, ...)