BUG#20397: Crash at ALTER TABLE t1 engine = x; for partitioned table

mysql-test/r/partition_error.result:
  New test cases
mysql-test/t/partition_error.test:
  New test cases
sql/sql_table.cc:
  ALTER TABLE t1 engine = x;
  will behave exactly like
  ALTER TABLE t1;
  See no reason why one should change handler to something not specified in query when
  there is already something one is attached to.

mysql-test/r/partition_error.result

 drop table if exists t1;
+create table t1 (a int)
+engine = x
+partition by key (a);
+Warnings:
+Error	1286	Unknown table engine 'x'
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1 PARTITION BY KEY (a) 
+drop table t1;
+create table t1 (a int)
+engine = innodb
+partition by list (a)
+(partition p0 values in (0));
+alter table t1 engine = x;
+Warnings:
+Error	1286	Unknown table engine 'x'
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL
+) ENGINE=InnoDB DEFAULT CHARSET=latin1 PARTITION BY LIST (a) (PARTITION p0 VALUES IN (0) ENGINE = InnoDB)
+drop table t1;
 partition by list (a)
 partitions 3
 (partition x1 values in (1,2,9,4) tablespace ts1,

mysql-test/t/partition_error.test

@@ -8,6 +8,24 @@
 drop table if exists t1;
 --enable_warnings
 
+#
+# Bug 20397: Partitions: Crash when using non-existing engine
+#
+create table t1 (a int)
+engine = x
+partition by key (a);
+show create table t1;
+drop table t1;
+
+create table t1 (a int)
+engine = innodb
+partition by list (a)
+(partition p0 values in (0));
+
+alter table t1 engine = x;
+show create table t1;
+drop table t1;
+
 #
 # Partition by key stand-alone error
 #
@@ -775,3 +793,5 @@ partition by range (a + (select count(*) from t1))
 -- error ER_PARTITION_FUNC_NOT_ALLOWED_ERROR
 create table t1 (a char(10))
 partition by hash (extractvalue(a,'a'));
+
+

sql/sql_table.cc

@@ -4992,7 +4992,19 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
 
   old_db_type= table->s->db_type;
   if (!create_info->db_type)
+  {
+    if (create_info->used_fields & HA_CREATE_USED_ENGINE)
+    {
+      /*
+        This case happens when the user specified
+        ENGINE = x where x is a non-existing storage engine
+        We clear the flag and treat it the same way
+        as if no storage engine was specified.
+      */
+      create_info->used_fields^= HA_CREATE_USED_ENGINE;
+    }
     create_info->db_type= old_db_type;
+  }
 
 #ifdef WITH_PARTITION_STORAGE_ENGINE
   if (prep_alter_part_table(thd, table, alter_info, create_info, old_db_type,