fixed error detection

sql/item_subselect.cc

@@ -1094,7 +1094,7 @@ int subselect_indexsubquery_engine::exec()
 	  *tab->null_ref_key= 1;
 	  null_finding= 1;
 	  /* Check if there exists a row with a null value in the index */
-	  if ((error= safe_index_read(tab)))
+	  if ((error= (safe_index_read(tab) == 1)))
 	    break;
 	}
       }