buffer: ignore negative allocation lengths

Treat negative length arguments to `Buffer()`/`allocUnsafe()`
as if they were zero so the allocation does not affect the
pool’s offset.

Fixes: https://github.com/nodejs/node/issues/7047
PR-URL: https://github.com/nodejs/node/pull/7051
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>

lib/buffer.js

@@ -199,8 +199,8 @@ Object.setPrototypeOf(SlowBuffer, Uint8Array);
 
 
 function allocate(size) {
-  if (size === 0) {
-    return createBuffer(size);
+  if (size <= 0) {
+    return createBuffer(0);
   }
   if (size < (Buffer.poolSize >>> 1)) {
     if (size > (poolSize - poolOffset))

test/parallel/test-buffer.js

@@ -1465,3 +1465,14 @@ assert.equal(Buffer.prototype.parent, undefined);
 assert.equal(Buffer.prototype.offset, undefined);
 assert.equal(SlowBuffer.prototype.parent, undefined);
 assert.equal(SlowBuffer.prototype.offset, undefined);
+
+{
+  // Test that large negative Buffer length inputs don't affect the pool offset.
+  assert.deepStrictEqual(Buffer(-Buffer.poolSize), Buffer.from(''));
+  assert.deepStrictEqual(Buffer(-100), Buffer.from(''));
+  assert.deepStrictEqual(Buffer.allocUnsafe(-Buffer.poolSize), Buffer.from(''));
+  assert.deepStrictEqual(Buffer.allocUnsafe(-100), Buffer.from(''));
+
+  // Check pool offset after that by trying to write string into the pool.
+  assert.doesNotThrow(() => Buffer.from('abc'));
+}