Fix a bug in test_c_api() that caused a negative refcount.

Objects/setobject.c

@@ -2205,7 +2205,7 @@ test_c_api(PySetObject *so)
 	Py_ssize_t count;
 	char *s;
 	Py_ssize_t i;
-	PyObject *elem, *dup, *t, *f, *dup2;
+	PyObject *elem=NULL, *dup=NULL, *t, *f, *dup2, *x;
 	PyObject *ob = (PyObject *)so;
 
 	/* Verify preconditions and exercise type/size checks */
@@ -2251,8 +2251,8 @@ test_c_api(PySetObject *so)
 
 	/* Exercise direct iteration */
 	i = 0, count = 0;
-	while (_PySet_Next((PyObject *)dup, &i, &elem)) {
-		s = PyString_AsString(elem);
+	while (_PySet_Next((PyObject *)dup, &i, &x)) {
+		s = PyString_AsString(x);
 		assert(s && (s[0] == 'a' || s[0] == 'b' || s[0] == 'c'));
 		count++;
 	}