SBP: Potential overflow issues

[NachoPal]

Review potential overflow situations where operators +, -, * and / are used. E.g:https://github.com/CESSProject/cess/blob/main/c-pallets/file-bank/src/lib.rs#L550

If operations are safe it is a good practice to add a comment about why.

Use checked_add, checked_sub, checked_mul, checked_div, instead.

[swowk]

thank you for your advice, w'll look into it. cc @shakawwq

[shakawwq]

The modification has been completed in commit https://github.com/CESSProject/cess/commit/6fa35a7f04806907084465401dc705ce5e616a1b. Thank you.