soroban-fixed-point-math has Incorrect Rounding and Overflow Handling in Signed Fixed-Point Math with Negatives

Summary

soroban-fixed-point-math is a fixed-point math library for Soroban smart contacts. In versions 1.3.0 and 1.4.0, the `mulDiv(x, y, z)` function incorrectly handled cases where both the intermediate product $x * y$ and the divisor $z$ were negative. The logic assumed that if the intermediate product was negative, the final result must also be negative, neglecting the sign of $z$. This resulted in rounding being applied in the wrong direction for cases where both $x * y$ and $z$ were negative. The functions most at risk are `fixed_div_floor` and `fixed_div_ceil`, as they often use non-constant numbers as the divisor $z$ in `mulDiv`. This error is present in all signed `FixedPoint` and `SorobanFixedPoint` implementations, including `i64`, `i128`, and `I256`. Versions 1.3.1 and 1.4.1 contain a patch. No known workarounds for this issue are available.