serde: fix overflow on i64::MIN

[vmx]

When decoding the minimum value of i64 (-9223372036854775808), there is an overflow error. With this fix it can be deserialized correctly.

The reason for the failure was the order of the operations. Prior to this change the decoding had these steps:

1. Decode the bytes into a u64 => 9223372036854775807
2. Add 1 => 9223372036854775808
3. Cast to i64 => error as i64::MAX is 9223372036854775807.
4. Negate the number

The new steps are:

1. Decode the bytes into a u64 => 9223372036854775807
2. Cast to i64 => 9223372036854775807
3. Negate the number => -9223372036854775807
4. Subtract 1 => -9223372036854775808

[quininer]

Thank you!