MDEV-18783 - Server crash in hp_rb_make_key

In debug build, whenever MEMORY table instance gets closed it performs
consistency check without protection. It may cause server crash if
executed concurrently with DML.

Moved consistency check to ha_heap::external_lock(F_UNLCK), so that it
is protected by THR_LOCK.

storage/heap/ha_heap.cc

@@ -453,6 +453,10 @@ int ha_heap::reset_auto_increment(ulonglong value)
 
 int ha_heap::external_lock(THD *thd, int lock_type)
 {
+#ifndef DBUG_OFF
+  if (lock_type == F_UNLCK && file->s->changed && heap_check_heap(file, 0))
+    return HA_ERR_CRASHED;
+#endif
   return 0;					// No external locking
 }
 

storage/heap/hp_close.c

@@ -35,12 +35,6 @@ int hp_close(register HP_INFO *info)
 {
   int error=0;
   DBUG_ENTER("hp_close");
-#ifndef DBUG_OFF
-  if (info->s->changed && heap_check_heap(info,0))
-  {
-    error=my_errno=HA_ERR_CRASHED;
-  }
-#endif
   info->s->changed=0;
   if (info->open_list.data)
     heap_open_list=list_delete(heap_open_list,&info->open_list);