Fix for GitHub issue #18 : undef propagation of const multiplies is i…

…ncorrect.

When an expression is elaborated, the compiler converts multiplies with
one constamt zero operand into a constant zero value. This is only valid
if the other operand is not a 4-state variable.

elab_expr.cc

@@ -538,7 +538,7 @@ NetExpr* PEBinary::elaborate_expr_base_mult_(Design*,
 		  return tmp;
 	    }
 
-	    if (rp_val.is_zero()) {
+	    if (rp_val.is_zero() && (lp->expr_type() != IVL_VT_LOGIC)) {
 		  NetEConst*tmp = make_const_0(expr_wid);
                   tmp->cast_signed(signed_flag_);
                   tmp->set_line(*this);