buffer: don't abort on prototype getters

Accessing prototype properties directly on a typed array will throw. So
do an extra check in Buffer's own getters to verify it is being called
on an instance.

Fixes: nodejs#3297

lib/buffer.js

@@ -304,6 +304,8 @@ Buffer.byteLength = byteLength;
 Object.defineProperty(Buffer.prototype, 'parent', {
   enumerable: true,
   get: function() {
+    if (!(this instanceof Buffer))
+      return undefined;
     if (this.byteLength === 0 ||
         this.byteLength === this.buffer.byteLength) {
       return undefined;
@@ -314,6 +316,8 @@ Object.defineProperty(Buffer.prototype, 'parent', {
 Object.defineProperty(Buffer.prototype, 'offset', {
   enumerable: true,
   get: function() {
+    if (!(this instanceof Buffer))
+      return undefined;
     return this.byteOffset;
   }
 });

test/parallel/test-buffer.js

@@ -1224,3 +1224,10 @@ assert.throws(function() {
 assert.throws(function() {
   new Buffer(null);
 }, /must start with number, buffer, array or string/);
+
+
+// Test prototype getters don't throw
+assert.equal(Buffer.prototype.parent, undefined);
+assert.equal(Buffer.prototype.offset, undefined);
+assert.equal(SlowBuffer.prototype.parent, undefined);
+assert.equal(SlowBuffer.prototype.offset, undefined);