MDEV-13982 Server crashes in in ha_partition::engine_name

table might be NULL in ha_partition::engine_name()
(test case pushed into 5.5)

sql/ha_partition.h

@@ -558,8 +558,7 @@ class ha_partition :public handler
   virtual THR_LOCK_DATA **store_lock(THD * thd, THR_LOCK_DATA ** to,
 				     enum thr_lock_type lock_type);
   virtual int external_lock(THD * thd, int lock_type);
-  LEX_CSTRING *engine_name()
-  { return hton_name(table->part_info->default_engine_type); }
+  LEX_CSTRING *engine_name() { return hton_name(partition_ht()); }
   /*
     When table is locked a statement is started by calling start_stmt
     instead of external_lock