Overflow/underflow handling (15 - Minor)

[ansermino]

Substrate does not have default overflow/underflow protection. In order to avoid this kind of bugs, it’s better to use safe functions like checked_add() and checked_sub(). In the current codebase math operations are mostly either incrementing (+1) or making operations with values that shouldn’t overflow/underflow logically:

chainbridge/src/lib.rs:L64

} else if self.votes_against.len() > (total - threshold) as usize {

It’s not really a big issue here, but if the threshold is bigger than the total, we’ll have underflow.