ln panics on Decimal::ZERO

[SebRollen]

MWE:

use rust_decimal::prelude::*;

fn main() {
    Decimal::ZERO.ln();
}

thread 'main' panicked at 'Division by zero', /Users/rollen/.cargo/registry/src/github.com-1ecc6299db9ec823/rust_decimal-1.14.2/src/decimal.rs:1972:45
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Of course taking the log of zero should yield some sort of error, but so should taking the log of negative numbers most likely, and for those the result is set to zero.

Decimal::new(-1, 0).ln() == Decimal::ZERO //true

The easiest fix might be to extend the predicate on this line to also check for *self != Decimal::ZERO

rust-decimal/src/maths.rs

Line 328 in e346c59

if self.is_sign_positive() {

. Happy to open a PR for that