BUG#20397: Crash when alter table t1 engine = x;

Review fixes

sql/sql_table.cc

@@ -4993,16 +4993,19 @@ 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)
+    if (table->part_info &&
+        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.
+        We set create_info->db_type to default_engine_type
+        to ensure we don't change underlying engine type
+        due to a erroneously given engine name.
       */
-      create_info->used_fields^= HA_CREATE_USED_ENGINE;
+      create_info->db_type= table->part_info->default_engine_type;
     }
+    else
       create_info->db_type= old_db_type;
   }