Vivado simulation issue: multiplying "x" with "0"

[sgherbst]

Looks like the Vivado simulator outputs "x" when "x" is multiplied by "0". For that reason, we might want to have mul_real check for that condition and output "0" if either input is zero, e.g.

assign prod = ((a == 0) || (b == 0)) ? 0 : (a * b);

This can otherwise cause x's to propagate during some startup scenarios.