fix signed parsing

bip/src/lib.rs

@@ -112,12 +112,10 @@ impl Symbol {
                     let v = Symbol::take(&mut it, |c| c.is_numeric());
 
                     if let Ok(n) = v.iter().collect::<String>().parse::<u64>() {
-                        let n = (n as i64 * sign) as u64;
-
                         syms.push(if sign > 0 {
                             Self::Int(n)
                         } else {
-                            Self::SInt(n)
+                            Self::SInt((n as i64 * sign) as u64)
                         });
                     } else {
                         return Err("invalid integer".to_string());