MDEV-21373 DBUG compilation - bad synchronization in ha_heap::external_lock()

ha_heap::external_lock contains some consistency checks for the table,#
in a debug compilation.

This code suffers from lack of synchronization, in a rare case
where mysql_lock_tables() fail, and unlock is forced, even if lock was
not previously taken.

To workaround, require EXTRA_DEBUG compile definition in order to activate
the consistency checks.The code still might be useful in some cases - but
the audience are developers looking for errors in single-threaded scenarios,
rather than multiuser stress-tests.

storage/heap/ha_heap.cc

@@ -423,7 +423,7 @@ int ha_heap::reset_auto_increment(ulonglong value)
 
 int ha_heap::external_lock(THD *thd, int lock_type)
 {
-#ifndef DBUG_OFF
+#if !defined(DBUG_OFF) && defined(EXTRA_DEBUG)
   if (lock_type == F_UNLCK && file->s->changed && heap_check_heap(file, 0))
     return HA_ERR_CRASHED;
 #endif