Prevent a few integer overflows

[nical]

Connections

Found while running the CTS in Firefox.

Description

We have a few places with the following pattern:

let last_foo = first_foo + foo_count;
if last_foo > foo_limit {
   // error ...
}

With the addition in the first line vulnerable to integer overflows. This PR addresses it by making sure that the limit is always smaller than u32::MAX and using saturating_add so that cases that would have overflown are treated as errors.

Testing

Covered by the CTS.

Checklist

• Run cargo fmt.
• Run cargo clippy.
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.