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

@@ -474,8 +474,11 @@ function _close(engine, callback) {
 
   engine._closed = true;
 
+  // Caller may invoke .close after a zlib error (which will null _handle).
+  if (engine._handle) {
     engine._handle.close();
   }
+}
 
 function emitCloseNT(self) {
   self.emit('close');