fix negative zero from multiplication/division by zero bigint

[mhaddon]

(-1) * 0 was producing a "negative zero" where Sign() returns -1 but String() returns "0". This breaks Cmp() comparisons.

addInline already handles this correctly - just needed the same fix in mulInline, quoInline, and remInline.

result := new(apd.BigInt).Mul(apd.NewBigInt(-1), apd.NewBigInt(0))

result.String() // "0" 
result.Sign()   // -1 (should be 0)
result.Cmp(apd.NewBigInt(0)) // -1 (should be 0)

Same issue with division and remainder when the result is zero.

Added the same zero check to all three functions. Existing tests pass.

[michae2]

Thanks for the fix! Would you be willing to add a short test to bigint_test.go?

[michae2]

nit: This could be shortened to neg := xNeg != yNeg && lo != 0 (and same with a couple others).

[mhaddon]

Is that better?

[michae2]

Looks great! Thank you!

Assuming tests pass, I'll go ahead and merge.