Clear greenlet weakrefs first

Otherwise user code may resurrect greenlet when
it is already destroyed, making references to
freed objects possible (see the test case).

greenlet.c

@@ -820,14 +820,14 @@ static void green_dealloc(PyGreenlet* self)
 			goto green_dealloc_end;
 		}
 	}
+	if (self->weakreflist != NULL)
+		PyObject_ClearWeakRefs((PyObject *) self);
 	Py_CLEAR(self->parent);
+	Py_CLEAR(self->run_info);
 	Py_CLEAR(self->exc_type);
 	Py_CLEAR(self->exc_value);
 	Py_CLEAR(self->exc_traceback);
 	Py_CLEAR(self->dict);
-	if (self->weakreflist != NULL)
-		PyObject_ClearWeakRefs((PyObject *) self);
-	Py_CLEAR(self->run_info);
 	Py_TYPE(self)->tp_free((PyObject*) self);
 green_dealloc_end:
 #ifdef GREENLET_USE_GC

tests/test_weakref.py

@@ -18,3 +18,17 @@ def test_inactive_weakref(self):
         o = weakref.ref(greenlet.greenlet())
         gc.collect()
         self.assertEqual(o(), None)
+
+    def test_dealloc_weakref(self):
+        seen = []
+        def worker():
+            try:
+                greenlet.getcurrent().parent.switch()
+            finally:
+                seen.append(g())
+        g = greenlet.greenlet(worker)
+        g.switch()
+        g2 = greenlet.greenlet(lambda: None, g)
+        g = weakref.ref(g2)
+        g2 = None
+        self.assertEqual(seen, [None])