bpo-33391: Fix refleak in set_symmetric_difference (GH-6670)

(cherry picked from commit 491bbedc)
Co-authored-by: lekma <lekmalek@gmail.com>

Misc/NEWS.d/next/Core and Builtins/2018-05-02-08-36-03.bpo-33391.z4a7rb.rst

+Fix a leak in set_symmetric_difference().

Objects/setobject.c

@@ -1710,8 +1710,10 @@ set_symmetric_difference(PySetObject *so, PyObject *other)
     if (otherset == NULL)
         return NULL;
     rv = set_symmetric_difference_update(otherset, (PyObject *)so);
-    if (rv == NULL)
+    if (rv == NULL) {
+        Py_DECREF(otherset);
         return NULL;
+    }
     Py_DECREF(rv);
     return (PyObject *)otherset;
 }