fix: prevent a possible infinite loop in WidgetDecoration.base_widget

[rsekman]

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Description:

WidgetDecoration.base_widget had a possible infinite loop if at any point the _original_widget points to somewhere higher up in the stack. See #879 (comment)

[coveralls]

Pull Request Test Coverage Report for Build 9075388171

Details

• 3 of 4 (75.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 74.687%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
urwid/widget/widget_decoration.py	3	4	75.0%

Totals
Change from base Build 8873706490:	0.002%
Covered Lines:	9396
Relevant Lines:	12726

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💛 - Coveralls