buffer: SlowBuffer only accept valid numeric values

Fixes a regression that appeared with the new Buffer implementation in v3.
Without this change, calling the SlowBuffer constructor with something else
than a number would abort on the C++ side. This makes sure that the length
argument is coerced to number or is 0.

Fixes: nodejs#2634
PR-URL: nodejs#2635
Reviewed-By: trevnorris - Trevor Norris <trev.norris@gmail.com>
Reviewed-By: cjihrig - Colin Ihrig <cjihrig@gmail.com>

lib/buffer.js

@@ -58,9 +58,9 @@ createPool();
 
 
 function SlowBuffer(length) {
-  if (length < 0)
+  if (+length != length)
     length = 0;
-  return binding.create(length);
+  return binding.create(+length);
 };
 
 SlowBuffer.prototype.__proto__ = Buffer.prototype;

test/parallel/test-buffer-slow.js

@@ -0,0 +1,55 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const buffer = require('buffer');
+const Buffer = buffer.Buffer;
+const SlowBuffer = buffer.SlowBuffer;
+
+const ones = [1, 1, 1, 1];
+
+// should create a Buffer
+let sb = new SlowBuffer(4);
+assert(sb instanceof Buffer);
+assert.strictEqual(sb.length, 4);
+sb.fill(1);
+assert.deepEqual(sb, ones);
+
+// underlying ArrayBuffer should have the same length
+assert.strictEqual(sb.buffer.byteLength, 4);
+
+// should work without new
+sb = SlowBuffer(4);
+assert(sb instanceof Buffer);
+assert.strictEqual(sb.length, 4);
+sb.fill(1);
+assert.deepEqual(sb, ones);
+
+// should work with edge cases
+assert.strictEqual(SlowBuffer(0).length, 0);
+try {
+  assert.strictEqual(SlowBuffer(buffer.kMaxLength).length, buffer.kMaxLength);
+} catch (e) {
+  assert.equal(e.message, 'Buffer allocation failed - process out of memory');
+}
+
+// should work with number-coercible values
+assert.strictEqual(SlowBuffer('6').length, 6);
+assert.strictEqual(SlowBuffer(true).length, 1);
+
+// should create zero-length buffer if parameter is not a number
+assert.strictEqual(SlowBuffer().length, 0);
+assert.strictEqual(SlowBuffer(NaN).length, 0);
+assert.strictEqual(SlowBuffer({}).length, 0);
+assert.strictEqual(SlowBuffer('string').length, 0);
+
+// should throw with invalid length
+assert.throws(function() {
+  new SlowBuffer(Infinity);
+}, 'invalid Buffer length');
+assert.throws(function() {
+  new SlowBuffer(-1);
+}, 'invalid Buffer length');
+assert.throws(function() {
+  new SlowBuffer(buffer.kMaxLength + 1);
+}, 'invalid Buffer length');