PyType_IsSubtype(): test tp_flags for HAVE_CLASS bit before accessing

PyType_IsSubtype(): test tp_flags for HAVE_CLASS bit before accessing
a->tp_mro.  If a doesn't have class, it's considered a subclass only
of itself or of 'object'.

This one fix is enough to prevent the ExtensionClass test suite from
dumping core, but that doesn't say much (it's a rather small test
suite).  Also note that for ExtensionClass-defined types, a different
subclass test may be needed.  But I haven't checked whether
PyType_IsSubtype() is actually used in situations where this matters
-- probably it doesn't, since we also don't check for classic classes.