bpo-36430: Fix a possible reference leak in itertools.count(). (GH-12551) (GH-12554)

(cherry picked from commit 0523c39e)
Co-authored-by: Zackery Spytz <zspytz@gmail.com>

Misc/NEWS.d/next/Core and Builtins/2019-03-25-23-37-26.bpo-36430.sd9xxQ.rst

+Fix a possible reference leak in :func:`itertools.count`.

Modules/itertoolsmodule.c

@@ -3321,6 +3321,7 @@ count_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     lz = (countobject *)type->tp_alloc(type, 0);
     if (lz == NULL) {
         Py_XDECREF(long_cnt);
+        Py_DECREF(long_step);
         return NULL;
     }
     lz->cnt = cnt;