Timestamp::nanoseconds panics when created from tendermint::Time with negative values

[Farhad-Shabani]

Summary

The Timestamp::nanoseconds() method returns a u64 value, assuming that a Timestamp with a negative UNIX timestamp is never constructed. While Timestamp's own methods prevent this, a Timestamp can still be created from tendermint::Time.
The tendermint::Time struct can be constructed using the from_unix_timestamp method, which accepts an i64 for seconds. This allows negative values to be passed. As a result, Timestamp::nanoseconds() can panic when created from a tendermint::Time with negative values.

Version

<= v0.53.0