buffer: revert length and parent check

In a rush to implement the fix 35e0d60 I overlooked the logic that
causes 0-length buffer instantiation to automatically not assign the
parent regardless.

lib/buffer.js

@@ -83,12 +83,10 @@ function Buffer(subject, encoding) {
   if (this.length < Buffer.poolSize / 2 && this.length > 0) {
     if (this.length > poolSize - poolOffset)
       createPool();
-    var parent = sliceOnto(allocPool,
-                           this,
-                           poolOffset,
-                           poolOffset + this.length);
-    if (this.length > 0)
-      this.parent = parent;
+    this.parent = sliceOnto(allocPool,
+                            this,
+                            poolOffset,
+                            poolOffset + this.length);
     poolOffset += this.length;
   } else {
     alloc(this, this.length);