Fix usize overflow

[suzp1984]

Fix usize overflow problem when calculate middle of two usize

Description

// left: usize;
// right: usize;
// left <= right;
let middle = (left + right) / 2;

if (left + right) > std::usize::MAX, then it's an overflow bug.

The common solution to calculate the middle value is:

let middle = left + (right - left) / 2;

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I ran bellow commands using the latest version of rust nightly.
• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I checked COUNTRIBUTING.md and my code follows its guidelines.

Please make sure that if there is a test that takes too long to run ( > 300ms), you #[ignore] that or
try to optimize your code or make the test easier to run. We have this rule because we have hundreds of
tests to run; If each one of them took 300ms, we would have to wait for a long time.

[siriak]

Looks good, thanks!