BUG#20583: index_last causes crash when performed on single partition

mysql-test/r/partition.result:
  New test case
mysql-test/t/partition.test:
  New test case
sql/ha_partition.cc:
  Ensure index_last always uses ordered index scan

mysql-test/r/partition.result

@@ -1043,4 +1043,12 @@ alter table t1 add partition (partition p2 values in (3));
 alter table t1 drop partition p2;
 use test;
 drop database db99;
+create table t1 (a int, index(a))
+partition by hash(a);
+insert into t1 values (1),(2);
+select * from t1 ORDER BY a DESC;
+a
+2
+1
+drop table t1;
 End of 5.1 tests

mysql-test/t/partition.test

@@ -1199,4 +1199,13 @@ alter table t1 drop partition p2;
 use test;
 drop database db99;
 
+#
+# Bug 20583 Partitions: Crash using index_last
+#
+create table t1 (a int, index(a))
+partition by hash(a);
+insert into t1 values (1),(2);
+select * from t1 ORDER BY a DESC;
+drop table t1;
+
 --echo End of 5.1 tests

sql/ha_partition.cc

@@ -3399,7 +3399,8 @@ int ha_partition::common_first_last(byte *buf)
 
   if ((error= partition_scan_set_up(buf, FALSE)))
     return error;
-  if (!m_ordered_scan_ongoing)
+  if (!m_ordered_scan_ongoing &&
+      m_index_scan_type != partition_index_last)
     return handle_unordered_scan_next_partition(buf);
   return handle_ordered_index_scan(buf);
 }