Handle 0-tuples which can be singletons.

Modules/itertoolsmodule.c

@@ -1919,7 +1919,7 @@ product_next(productobject *lz)
 			Py_DECREF(old_result);
 		}
 		/* Now, we've got the only copy so we can update it in-place */
-		assert (Py_REFCNT(result) == 1);
+		assert (npools==0 || Py_REFCNT(result) == 1);
 
                 /* Update the pool indices right-to-left.  Only advance to the
                    next pool when the previous one rolls-over */