buffer: throw if both length and enc are passed

lib/buffer.js

@@ -47,6 +47,11 @@ function alignPool() {
 function Buffer(arg, encoding) {
   // Common case.
   if (typeof arg === 'number') {
+    if (typeof encoding === 'string') {
+      throw new Error(
+        'If encoding is specified then the first argument must be a string'
+      );
+    }
     // If less than zero, or NaN.
     if (arg < 0 || arg !== arg)
       arg = 0;

test/sequential/test-buffer-bad-overload.js

@@ -0,0 +1,15 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+
+assert.doesNotThrow(function() {
+  new Buffer(10);
+});
+
+assert.throws(function() {
+  new Buffer(10, 'hex');
+});
+
+assert.doesNotThrow(function() {
+  new Buffer('deadbeaf', 'hex');
+});