[3.13] gh-124064: Fix -Wconversion warnings in Parser/string_parser.c (GH-124204) (#124216)

gh-124064: Fix -Wconversion warnings in Parser/string_parser.c (GH-124204)

Fix integer overflow check in decode_unicode_with_escapes(): use
PY_SSIZE_T_MAX instead of SIZE_MAX.
(cherry picked from commit f9fa6ba4f8d90ae12bc1f6a792d66903bb169ba8)

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