zlib: release callback and buffer after processing

PR-URL: #6955
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Yuval Brik <yuval@brik.org.il>

lib/zlib.js

@@ -560,6 +560,16 @@ Zlib.prototype._processChunk = function(chunk, flushFlag, cb) {
   req.callback = callback;
 
   function callback(availInAfter, availOutAfter) {
+    // When the callback is used in an async write, the callback's
+    // context is the `req` object that was created. The req object
+    // is === this._handle, and that's why it's important to null
+    // out the values after they are done being used. `this._handle`
+    // can stay in memory longer than the callback and buffer are needed.
+    if (this) {
+      this.buffer = null;
+      this.callback = null;
+    }
+
     if (self._hadError)
       return;