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

(cherry picked from commit 0523c39)

Co-authored-by: Zackery Spytz <[email protected]>

Author: miss-islington

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

@@ -0,0 +1 @@
+Fix a possible reference leak in :func:`itertools.count`.

Modules/itertoolsmodule.c

@@ -4027,6 +4027,7 @@ 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;