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: #7047

Refs: #7051
PR-URL: #7221
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/buffer.js

@@ -163,8 +163,8 @@ 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

@@ -1438,3 +1438,14 @@ 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'));
+}