Fix galera test failures.

sql/sql_class.cc

@@ -2177,7 +2177,7 @@ bool THD::notify_shared_lock(MDL_context_owner *ctx_in_use,
 #ifdef WITH_WSREP
         {
           signalled|= mysql_lock_abort_for_thread(this, thd_table);
-          if (WSREP(this) && wsrep_thd_is_BF((void *)this, FALSE))
+          if (this && WSREP(this) && wsrep_thd_is_BF((void *)this, FALSE))
           {
             WSREP_DEBUG("remove_table_from_cache: %llu",
                         (unsigned long long) this->real_id);