MDEV-35849: index records in a wrong order

page_cur_dtuple_cmp(): Take DESC index fields into account also when
comparing a NULL value in a ROW_FORMAT≠REDUNDANT record.

Thanks to Elena Stepanova for finding this bug.

mysql-test/suite/innodb/r/innodb-index.result

@@ -1996,4 +1996,14 @@ a
 c
 DROP TABLE t1;
 # End of 10.8 tests
+#
+# MDEV-35849: index records in a wrong order
+#
+CREATE TABLE t (pk INT PRIMARY KEY, a VARCHAR(1), KEY (a DESC)) ENGINE=InnoDB;
+INSERT INTO t VALUES (1,NULL),(2,'x');
+CHECK TABLE t;
+Table	Op	Msg_type	Msg_text
+test.t	check	status	OK
+DROP TABLE t;
+# End of 11.8 tests
 ALTER DATABASE test CHARACTER SET utf8mb4 COLLATE utf8mb4_uca1400_ai_ci;

mysql-test/suite/innodb/t/innodb-index.test

@@ -1229,6 +1229,17 @@ DROP TABLE t1;
 
 --echo # End of 10.8 tests
 
+--echo #
+--echo # MDEV-35849: index records in a wrong order
+--echo #
+
+CREATE TABLE t (pk INT PRIMARY KEY, a VARCHAR(1), KEY (a DESC)) ENGINE=InnoDB;
+INSERT INTO t VALUES (1,NULL),(2,'x');
+CHECK TABLE t;
+DROP TABLE t;
+
+--echo # End of 11.8 tests
+
 --disable_query_log
 
 call mtr.add_suppression("InnoDB: Tablespace .* was not found at .*t[12].ibd.");

storage/innobase/page/page0cur.cc

@@ -606,7 +606,7 @@ static int page_cur_dtuple_cmp(const dtuple_t &dtuple, const rec_t *rec,
         {
           if (i < cur_field || dtuple.fields[i].len == UNIV_SQL_NULL)
             continue;
-          ret= 1;
+          ret= field->descending ? -1 : 1;
           break;
         }
       }