zlib: fix linting recently-introduced lint error

Remove unnecessary named function. V8 will do a better job inferring the
name from the assignment to a property. The current formulation does not
pass linting.

PR-URL: nodejs#9524
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/zlib.js

@@ -346,7 +346,7 @@ function Zlib(opts, mode) {
 
   var self = this;
   this._hadError = false;
-  this._handle.onerror = function onErrorHandler(message, errno) {
+  this._handle.onerror = function(message, errno) {
     // there is no way to cleanly recover.
     // continuing only obscures problems.
     _close(self);