buffer: stop alloc() uninitialized memory return

CVE-2018-7166
Discovered by ChALkeR - Сковорода Никита Андреевич

Prevent Buffer.alloc(size, fill, number) from returning uninitialized memory.

Fixes: https://github.com/nodejs-private/security/issues/202
PR-URL: https://github.com/nodejs-private/node-private/pull/137
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

lib/buffer.js

@@ -278,7 +278,8 @@ function assertSize(size) {
 Buffer.alloc = function alloc(size, fill, encoding) {
   assertSize(size);
   if (fill !== undefined && fill !== 0 && size > 0) {
-    return _fill(createUnsafeBuffer(size), fill, encoding);
+    const buf = createUnsafeBuffer(size);
+    return _fill(buf, fill, 0, buf.length, encoding);
   }
   return new FastBuffer(size);
 };

test/parallel/test-buffer-alloc.js

@@ -1039,3 +1039,10 @@ common.expectsError(() => {
   code: 'ERR_INVALID_ARG_VALUE',
   type: TypeError
 });
+
+common.expectsError(() => {
+  Buffer.alloc(40, 'x', 20);
+}, {
+  code: 'ERR_INVALID_ARG_TYPE',
+  type: TypeError
+});