ptx: g flag should accept only positive non-zero values

uutils ptx currently accepts a zero -g value, GNU doesn't ### uutils ptx: ``` $ cargo run ptx -g 0 Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.11s Running `target/debug/coreutils ptx -g 0` ^C ``` ### GNU ptx: ``` $ ptx -g 0 ptx: invalid gap width: ‘0’ ```