Fix segfault after test failure, when Device used as a pytest fixture

Yury Matveevrequested to merge
652-segfault-when-an-exception-is-thrown-after-monkey-patching-a-device-class-in-a-pytest-fixture into develop

Closes #652 (closed)

It is a tricky case, so I, actually, cannot create a clear compact test for it, so I tried to simplify Tomase`s code as possible. Then I did implement Anton's suggestion (the first one, with checking Util) and it works.

This does not make use of Device as pytest fixture 100% safe, but should be better.

Edited by Yury Matveev

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