ceval.c's GETITEM should have asserts, not set exceptions

[sweeneyde]

Before this change: the following segfaults at Py_INCREF(value) in LOAD_CONST:

>>> f = lambda: 42
>>> f.__code__ = f.__code__.replace(co_consts=())
>>> f()

This is because PyTuple_GetItem raises and returns NULL, which is never checked for.

After the change:

>>> f = lambda: 42
>>> f.__code__ = f.__code__.replace(co_consts=())
>>> f()
Assertion failed: i < PyTuple_GET_SIZE(v), file C:\Users\sween\Source\Repos\cpython2\cpython\Python\ceval.c, line 740

Also, PyTuple_GET_ITEM already casts.

This is a change that only applies in debug mode to malformed code, so skipping issue/news.

[kumaraditya303]

LGTM, assertion failure is better than segfault.

[rhettinger]

Perhaps add assertions for the other two tests in PyTuple_GetItem():

assert(PyTuple_Check(v);
assert(Py_REFCNT(v) == 1);

[sweeneyde]

Added PyTuple_Check to be explicit, though I believe the macros do that in debug mode regardless.

It looks like the refcount assertion would only be for PyTuple_SetItem().

[rhettinger]

This looks like a nice improvement.