Check Symbol properly

decimal.js

@@ -4841,7 +4841,7 @@
 
   // Node and other environments that support module.exports.
   } else if (typeof module != 'undefined' && module.exports) {
-    if (Symbol && typeof Symbol.iterator == 'symbol') {
+    if (typeof Symbol == 'function' && typeof Symbol.iterator == 'symbol') {
       P[Symbol.for('nodejs.util.inspect.custom')] = P.toString;
       P[Symbol.toStringTag] = 'Decimal';
     }