[3.14] gh-146093: Fix csv _set_str(): check if PyUnicode_DecodeASCII() failed (GH-146113) (#146130)

gh-146093: Fix csv _set_str(): check if PyUnicode_DecodeASCII() failed (GH-146113)

The function can fail on a memory allocation failure.

Bug reported by devdanzin.
(cherry picked from commit 724c7c8146f44a7c737ec4588a1ee4b9db994f6f)

Co-authored-by: Victor Stinner <vstinner@python.org>