potential tx.verify fix.

lib/bcoin/tx.js

@@ -437,6 +437,9 @@ TX.prototype.verify = function verify(index, force) {
     if (!res)
       return false;
 
+    // Might be necessary for arithmetic:
+    // if (stack.length === 0 || new bn(stack.pop()).cmp(0) !== 0)
+
     if (stack.length === 0 || !utils.isEqual(stack.pop(), [ 1 ]))
       return false;