Fix a crash in cross-thread throw

greenlet.c

@@ -479,13 +479,13 @@ g_switch(PyGreenlet* target, PyObject* args, PyObject* kwargs)
 
 	/* check ts_current */
 	if (!STATE_OK) {
-		Py_DECREF(args);
+		Py_XDECREF(args);
 		Py_XDECREF(kwargs);
 		return NULL;
 	}
 	run_info = green_statedict(target);
 	if (run_info == NULL || run_info != ts_current->run_info) {
-		Py_DECREF(args);
+		Py_XDECREF(args);
 		Py_XDECREF(kwargs);
 		PyErr_SetString(PyExc_GreenletError, run_info
 				? "cannot switch to a different thread"

tests/test_greenlet.py

@@ -331,3 +331,16 @@ def __getattribute__(self, name):
                 return greenlet.__getattribute__(self, name)
         g = mygreenlet(lambda: None)
         self.assertRaises(SomeError, g.throw, SomeError())
+
+    def test_throw_doesnt_crash(self):
+        result = []
+        def worker():
+            greenlet.getcurrent().parent.switch()
+        def creator():
+            g = greenlet(worker)
+            g.switch()
+            result.append(g)
+        t = threading.Thread(target=creator)
+        t.start()
+        t.join()
+        self.assertRaises(greenlet.error, result[0].throw, SomeError())