[issue20929] Undefined behavior flagged by Clang 3.4

[Roundup Robot]

Roundup Robot added the comment:

New changeset e68f156ea0e6 by Benjamin Peterson in branch 'default':
cast negative numbers to size_t before shifting them (#20929)
http://hg.python.org/cpython/rev/e68f156ea0e6

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nosy: +python-dev

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue20929>
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