Segfault from py::str()

[davidhewitt]

In the sample below:

pybind11/include/pybind11/pytypes.h

Lines 928 to 936 in 25abf7e

	static PyObject *raw_str(PyObject *op) {
	PyObject *str_value = PyObject_Str(op);
	#if PY_MAJOR_VERSION < 3
	if (!str_value) throw error_already_set();
	PyObject *unicode = PyUnicode_FromEncodedObject(str_value, "utf-8", nullptr);
	Py_XDECREF(str_value); str_value = unicode;
	#endif
	return str_value;
	}

I'm pretty sure the #if PY_MAJOR_VERSION < 3 and if (!str_value) throw error_already_set(); lines are the wrong way around. As it stands, on Python 3 then if the PyObject_Str() call fails then a nullptr becomes the ptr held by a py::str object, leading to possible segfaults.