MDEV-6814: Server crashes in calculate_key_len on query with ORDER BY

- if test_if_skip_sort_order() decides to switch to using an index, or
  switch from using ref to using quick select, it should set all
  members accordingly.

mysql-test/r/order_by_optimizer_innodb.result

@@ -47,6 +47,14 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	range	PRIMARY,ux_pk1_fd5	ux_pk1_fd5	13	NULL	137	Using where
 drop table t0,t1, t2;
 #
+# MDEV-6814: Server crashes in calculate_key_len on query with ORDER BY
+#
+CREATE TABLE t1 (f1 INT, f2 INT, f3 INT, KEY(f2),KEY(f2,f1)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1,5,0),(2,6,0);
+SELECT * FROM t1 WHERE f1 < 3 AND f2 IS NULL ORDER BY f1;
+f1	f2	f3
+DROP TABLE t1;
+#
 # MDEV-6796: Unable to skip filesort when using implicit extended key
 #
 CREATE TABLE t1 (

mysql-test/t/order_by_optimizer_innodb.test

@@ -42,6 +42,14 @@ EXPLAIN SELECT * FROM t2                       WHERE pk1=9 AND fd5 < 500 ORDER B
 
 drop table t0,t1, t2;
 
+--echo #
+--echo # MDEV-6814: Server crashes in calculate_key_len on query with ORDER BY
+--echo #
+CREATE TABLE t1 (f1 INT, f2 INT, f3 INT, KEY(f2),KEY(f2,f1)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1,5,0),(2,6,0);
+SELECT * FROM t1 WHERE f1 < 3 AND f2 IS NULL ORDER BY f1;
+DROP TABLE t1;
+
 --echo #
 --echo # MDEV-6796: Unable to skip filesort when using implicit extended key
 --echo #

sql/sql_select.cc

@@ -20177,6 +20177,7 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
           tab->ref.key= -1;
           tab->ref.key_parts= 0;
           tab->use_quick= 1;
+          best_key= new_ref_key;
           /*
             We don't restore select->cond as we want to use the
             original condition as index condition pushdown is not