zlib: fix use after null when calling .close

An internal zlib error may cause _handle to be set to null.
Close now will check if there is a _handle prior to calling .close on
it.

PR-URL: https://github.com/nodejs/node/pull/5982
Fixes: https://github.com/nodejs/node/issues/6034
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/zlib.js

@@ -462,7 +462,9 @@ Zlib.prototype.close = function(callback) {
 
   this._closed = true;
 
-  this._handle.close();
+  if (this._handle) {
+    this._handle.close();
+  }
 
   process.nextTick(emitCloseNT, this);
 };