Fix UB in when multiplying by zero

[mirthfulLee]

Summary

This PR fixes UB in SizeValue::mul when multiplication yields 0 (for example, rhs = 0).

Changes

• Replace Self(unsafe { NonZeroU64::new_unchecked(val) }) to Self::new(val)
• Add a regression test mul_zero_panic to ensure SizeValue::new(8).mul(0) panics.

Fix #108.

[cwfitzgerald]

Thank you! Main CI is failing unrelated to this change.