buffer: use FastBuffer when fill is set to 0

A large number of libraries seem to use Buffer.alloc(size, 0)
instead of just Buffer.alloc(size).

We don't need to follow the "create unsafe buffer and fill it" path
(i.e. actually allocate and perform fill) in that situation, that is
better handled by Uint8Array constructor.

Buffer.alloc(size) and Buffer.alloc(size, 0) are equivalent, so
use the same code path.

Not performing the zero-fill manually and having the underlying memory
allocator do it for us can improve speed and reduce the memory usage
for situations where Buffer.alloc(size, 0) is used.

PR-URL: #21989
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: Minwoo Jung <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Tiancheng "Timothy" Gu <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>

lib/buffer.js

@@ -276,7 +276,7 @@ function assertSize(size) {
  */
 Buffer.alloc = function alloc(size, fill, encoding) {
   assertSize(size);
-  if (fill !== undefined && size > 0) {
+  if (fill !== undefined && fill !== 0 && size > 0) {
     return _fill(createUnsafeBuffer(size), fill, encoding);
   }
   return new FastBuffer(size);