`Decimal::from_str_radix` still panics on a long radix-36 value after #807

[SichangHe]

In a debug build, evaluating

Decimal::from_str_radix(
    "zzzzzzzzzzzzzzzzzzz.zzzzzzzzzzzzzzzzzzz",
    36,
)

panics with an unsigned subtraction underflow. It should instead return
Err(Error::ExceedsMaximumPossibleValue).

The input has 19 radix-36 digits before and after the decimal point.

The panic occurs after a rounding carry after an underflow here:

rust-decimal/src/str.rs

Line 713 in 6683079

(coeff.len() as u32) - (digits_before_dot as u32)

#807 did not fix this.