Fix a too-aggressive assert (see SF#1257960). Previously, gen_iternext

Fix a too-aggressive assert (see SF#1257960).  Previously, gen_iternext
was never called during interpreter shutdown GC, so the f_back!=NULL
assertion was correct.  Now that generators get close()d during GC,
the assertion was being triggered because the generator close() was being
called as the top-level frame.  However, nothing actually is broken by
this; it's just that the condition was unexpected in previous Python
versions.