[issue29438] use after free in key sharing dict

[INADA Naoki]

INADA Naoki added the comment:

> Why res == 0 is added? If PyDict_SetItem() triggers recursive calling of _PyObjectDict_SetItem() which calls PyDict_SetItem() it may be possible that the first PyDict_SetItem() is failed while the dict is changed by the second PyDict_SetItem() and CACHED_KEYS(tp) becomes outdated.

To avoid hiding error raised in PyDict_SetItem().
But it seems I was too nervous.  The error will be hidden only when make_keys_shared() raise exception.
I'll remove the check.

BTW, how about -py35.patch?  It is minimum patch to avoid "use after free".  It skip 
CACHED_KEYS(tp) = NULL entirely.  But I think I can apply same patch to Python 3.5 too.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue29438>
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