Issue python#117: fix a reference leak in stackless.threads

Fix two reference leaks in the C-implementation of stackless.threads.

https://bitbucket.org/stackless-dev/stackless/issues/117
(grafted from 0ed22581e9fc255195d9116446ab9b20bcf99ac8)

Stackless/changelog.txt

@@ -21,7 +21,8 @@ What's New in Stackless 3.X.X?
 - https://bitbucket.org/stackless-dev/stackless/issues/117
   Fix various reference leaks:
   - Leak of a reference to Py_None in generator.throw()
-
+  - Leak of a reference to the thread-id of every thread returned by stackless.threads
+
   Additionally this change brings the handling of caught exceptions more in
   line with C-Python.
 

Stackless/module/stacklessmodule.c

@@ -1524,9 +1524,16 @@ slpmodule_getthreads(PyObject *self)
 
     for (ts = interp->tstate_head; ts != NULL; ts = ts->next) {
         PyObject *id = PyLong_FromLong(ts->thread_id);
-
-        if (id == NULL || PyList_Append(lis, id))
+        if (id == NULL) {
+            Py_DECREF(lis);
+            return NULL;
+        }
+        if (PyList_Append(lis, id)) {
+            Py_DECREF(lis);
+            Py_DECREF(id);
             return NULL;
+        }
+        Py_DECREF(id);
     }
     PyList_Reverse(lis);
     return lis;