Fixed crash in failing instant alter table with partitioned table

MDEV-22649 SIGSEGV in ha_partition::create_partitioning_metadata on ALTER
MDEV-22804 SIGSEGV in ha_partition::create_partitioning_metadata

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

+set @save_alter_algorithm= @@session.alter_algorithm;
+SET SESSION alter_algorithm=4;
+CREATE TABLE t1(a INT) engine=myisam PARTITION BY RANGE(a) SUBPARTITION BY KEY(a) (PARTITION p0 VALUES LESS THAN (10) (SUBPARTITION s0,SUBPARTITION s1), PARTITION p1 VALUES LESS THAN (20) (SUBPARTITION s2,SUBPARTITION s3));
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+ PARTITION BY RANGE (`a`)
+SUBPARTITION BY KEY (`a`)
+(PARTITION `p0` VALUES LESS THAN (10)
+ (SUBPARTITION `s0` ENGINE = MyISAM,
+  SUBPARTITION `s1` ENGINE = MyISAM),
+ PARTITION `p1` VALUES LESS THAN (20)
+ (SUBPARTITION `s2` ENGINE = MyISAM,
+  SUBPARTITION `s3` ENGINE = MyISAM))
+ALTER TABLE t1 ADD COLUMN c INT;
+ERROR 0A000: ALGORITHM=INSTANT is not supported for this operation. Try ALGORITHM=COPY
+DROP table if exists t1;
+set @@session.alter_algorithm= @save_alter_algorithm;
+CREATE TABLE t1 (a INT) PARTITION BY RANGE(a) SUBPARTITION BY HASH(a) (PARTITION p VALUES LESS THAN (5) (SUBPARTITION sp, SUBPARTITION sp1), PARTITION p1 VALUES LESS THAN MAXVALUE (SUBPARTITION sp2, SUBPARTITION sp3));
+ALTER TABLE t1 DROP PARTITION p;
+DROP TABLE if exists t1;

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

+#
+# General errors with ALTER TABLE and partitions that doesn't have to be run
+# on all engines
+#
+
+--source include/have_partition.inc
+
+#
+# MDEV-22649 SIGSEGV in ha_partition::create_partitioning_metadata on ALTER
+#
+
+set @save_alter_algorithm= @@session.alter_algorithm;
+SET SESSION alter_algorithm=4;
+CREATE TABLE t1(a INT) engine=myisam PARTITION BY RANGE(a) SUBPARTITION BY KEY(a) (PARTITION p0 VALUES LESS THAN (10) (SUBPARTITION s0,SUBPARTITION s1), PARTITION p1 VALUES LESS THAN (20) (SUBPARTITION s2,SUBPARTITION s3));
+show create table t1;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED
+ALTER TABLE t1 ADD COLUMN c INT;
+DROP table if exists t1;
+set @@session.alter_algorithm= @save_alter_algorithm;
+
+
+#
+# MDEV-22804 SIGSEGV in ha_partition::create_partitioning_metadata |
+# ERROR 1507 (HY000): Error in list of partitions to DROP
+#
+
+CREATE TABLE t1 (a INT) PARTITION BY RANGE(a) SUBPARTITION BY HASH(a) (PARTITION p VALUES LESS THAN (5) (SUBPARTITION sp, SUBPARTITION sp1), PARTITION p1 VALUES LESS THAN MAXVALUE (SUBPARTITION sp2, SUBPARTITION sp3));
+ALTER TABLE t1 DROP PARTITION p;
+DROP TABLE if exists t1;

sql/ha_partition.cc

@@ -686,7 +686,9 @@ int ha_partition::create_partitioning_metadata(const char *path,
   if (m_part_info)
   {
     part= m_part_info->partitions.head();
-    if ((part->engine_type)->create_partitioning_metadata &&
+    /* part->engine_type may be 0 when we failed to create the partition */
+    if (part->engine_type &&
+        (part->engine_type)->create_partitioning_metadata &&
         ((part->engine_type)->create_partitioning_metadata)(path, old_path,
                                                             action_flag))
     {