Handle potential overflow when calculating fee to be substracted

[girazoki]

What does it do?

Probably not exploitable because the queue would prevent a message with that much weight from beign executed, but still

plus weight is u64

What important points reviewers should know?

Is there something left for follow-up PRs?

What alternative implementations were considered?

Are there relevant PRs or issues in other repositories (Substrate, Polkadot, Frontier, Cumulus)?

What value does it bring to the blockchain users?

[notlesh]

Looks good to me.

One other thing I would consider in cases like this is whether or not the max int size is sufficient to prevent spam. Obviously, it's better than an int overflow, but ideally the max int size is going to exhaust block space in all cases (in other words, be invalid).

[girazoki]

In this case they would need to pay a lot of money to even try to get this message processed. So in that sense I think we are good with this particular case