[3.13] gh-132769: Refactor possible read-out-of-bounds in `lexer.c` (GH-132770) (#132788)

gh-132769: Refactor possible read-out-of-bounds in `lexer.c` (GH-132770)
(cherry picked from commit ea8ec95cfadbf58a11ef8e41341254d982a1a479)

Co-authored-by: sobolevn <mail@sobolevn.me>