[security] bpo-13617: Reject embedded null characters in wchar* strings. (#2302)

Based on patch by Victor Stinner.

Add private C API function _PyUnicode_AsUnicode() which is similar to
PyUnicode_AsUnicode(), but checks for null characters.

Modules/spwdmodule.c

@@ -134,6 +134,7 @@ spwd_getspnam_impl(PyObject *module, PyObject *arg)
 
     if ((bytes = PyUnicode_EncodeFSDefault(arg)) == NULL)
         return NULL;
+    /* check for embedded null bytes */
     if (PyBytes_AsStringAndSize(bytes, &name, NULL) == -1)
         goto out;
     if ((p = getspnam(name)) == NULL) {