check coin arg is valid

lib/server.js

@@ -1322,7 +1322,7 @@ WalletService.prototype._getActiveAddresses = function(cb) {
 /**
  * Get wallet balance.
  * @param {Object} opts
- * @param {string} [opts.coin = 'btc'] - Override wallet coin.
+ * @param {string} [opts.coin] - Override wallet coin (default wallet's coin).
  * @param {Boolean} opts.twoStep[=false] - Optional - Use 2 step balance computation for improved performance
  * @returns {Object} balance - Total amount & locked amount.
  */
@@ -1331,6 +1331,11 @@ WalletService.prototype.getBalance = function(opts, cb) {
 
   opts = opts || {};
 
+  if (opts.coin) {
+    if (!Utils.checkValueInCollection(opts.coin, Constants.COINS))
+      return cb(new ClientError('Invalid coin'));
+  }
+
   if (!opts.twoStep)
     return self._getBalanceOneStep(opts, cb);