Repair refcounting on error return from type_set_bases.

Include a test case that failed for one of my efforts to repair this.

Lib/test/test_descr.py

@@ -3585,6 +3585,13 @@ def test_mutable_bases():
         # actually, we'll have crashed by here...
         raise TestFailed, "shouldn't be able to create inheritance cycles"
 
+    try:
+        D.__bases__ = (C, C)
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "didn't detect repeated base classes"
+
     try:
         D.__bases__ = (E,)
     except TypeError:

Objects/typeobject.c

@@ -303,13 +303,16 @@ type_set_bases(PyTypeObject *type, PyObject *value, void *context)
 	return r;
 
   bail:
+	Py_DECREF(type->tp_bases);
+	Py_DECREF(type->tp_base);
+	if (type->tp_mro != old_mro) {
+		Py_DECREF(type->tp_mro);
+	}
+
 	type->tp_bases = old_bases;
 	type->tp_base = old_base;
 	type->tp_mro = old_mro;
 
-	Py_DECREF(value);
-	Py_DECREF(new_base);
-
 	return -1;
 }