MDEV-21007 Do not assert auto_increment_value unless all parts open

Commit 6dce6aec breaks out of a loop
in ha_partition::info when some partitions aren't opened, in which
case auto_increment_value assertion will fail. This commit patches
that hole.

mysql-test/suite/parts/r/mdev_21007.result

+CREATE TABLE t1 (a INT) PARTITION BY RANGE (a) (PARTITION p0 VALUES LESS THAN (1), PARTITION p1 VALUES LESS THAN (MAXVALUE));
+INSERT INTO t1 VALUES (1),(2);
+ALTER TABLE t1 MODIFY a INT AUTO_INCREMENT PRIMARY KEY;
+UPDATE t1 PARTITION (p1) SET a=9 ORDER BY a LIMIT 1;
+DROP TABLE t1;

mysql-test/suite/parts/t/mdev_21007.test

+--source include/have_partition.inc
+ 
+CREATE TABLE t1 (a INT) PARTITION BY RANGE (a) (PARTITION p0 VALUES LESS THAN (1), PARTITION p1 VALUES LESS THAN (MAXVALUE));
+INSERT INTO t1 VALUES (1),(2);
+ALTER TABLE t1 MODIFY a INT AUTO_INCREMENT PRIMARY KEY;
+UPDATE t1 PARTITION (p1) SET a=9 ORDER BY a LIMIT 1;
+ 
+# Cleanup
+DROP TABLE t1;

sql/ha_partition.cc

@@ -8416,7 +8416,7 @@ int ha_partition::info(uint flag)
                         file->stats.auto_increment_value);
         } while (*(++file_array));
 
-        DBUG_ASSERT(auto_increment_value);
+        DBUG_ASSERT(!all_parts_opened || auto_increment_value);
         stats.auto_increment_value= auto_increment_value;
         if (all_parts_opened && auto_inc_is_first_in_idx)
         {