[issue13274] heapq pure python version uses islice without guarding for negative counts

[Éric Araujo]

Changes by Éric Araujo <merwok at netwok.org>:


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stage:  -> test needed
versions:  -Python 2.6, Python 3.1, Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue13274>
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