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: nodejs#7047

Refs: nodejs#7051
Refs: nodejs#7221
Refs: nodejs#7475
PR-URL: nodejs#7562
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Nikolai Vavilov <vvnicholas@gmail.com>

lib/buffer.js

@@ -79,8 +79,8 @@ 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))