don't let current_thd to point to a destroyed THD

* reset current_thd in THD::~THD, otherwise my_malloc_size_cb_func()
  might access THD after it was destroyed.
* remove now redundant set_current_thd(0) calls that follow delete thd.

sql/sql_plugin.cc

@@ -1775,7 +1775,6 @@ static void plugin_load(MEM_ROOT *tmp_root)
 end:
   /* Remember that we don't have a THD */
   delete new_thd;
-  set_current_thd(0);
   DBUG_VOID_RETURN;
 }