In theory you are right. m->md_def could be NULL, too. But in practice it's only going to happen when you have a faulty C extension. The code tries to load a dynamic module (ELF shared library, Windows DLL, ...) with _PyImport_GetDynLoadFunc() a couple of lines before PyModule_GetDef(). Any invalid file is rejected:

>>> imp.load_dynamic("os", "Lib/os.py")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: Lib/os.py: invalid ELF header

But an extra check doesn't hurt. How do you like this?

    def = PyModule_GetDef(m);
    if (def == NULL) {
        if (!PyErr_Occured()) {
            /* m->md_def == NULL */
            PyErr_BadArgument();
        }
        goto error;
    }