tendermint: ensure ValidatorIndex is ≤ i32::MAX

[mina86]

During conversion from usize to ValidatorIndex first convert the value to i32 to make sure that the value is ≤ i32::MAX. Without that intermediate conversion, u32::MAX as usize is successfully converted into the index.

• [ ] Referenced an issue explaining the need for the change
• [x] Updated all relevant documentation in docs
• [x] Updated all code comments where relevant
• [x] Wrote tests
• [x] Added entry in .changelog/