update_refs(): assert that incoming refcounts aren't 0. The comment

update_refs():  assert that incoming refcounts aren't 0.  The comment
for this function has always claimed that was true, but it wasn't
verified before.  For the latest batch of "double deallocation" bugs
(stemming from weakref callbacks invoked by way of subtype_dealloc),
this assert would have triggered (instead of waiting for
_Py_ForgetReference to die with a segfault later).