[issue27963] null poiter dereference in set_conversion_mode due uncheck _ctypes_conversion_errors

[Serhiy Storchaka]

Changes by Serhiy Storchaka <storchaka+cpython at gmail.com>:


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stage:  -> patch review

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27963>
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