gh-38432: fix memory leak in is_planar
Fixes the memory leak in `is_planar` reported at
https://ask.sagemath.org/question/78467/memory-leak-in-is_planar/
We now ensure that memory is released before returning the answer.
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preview.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/38432
Reported by: David Coudert
Reviewer(s): Travis Scrimshaw
