browser: return error for double end().

lib/db/level.js

@@ -144,6 +144,8 @@ Iterator.prototype.seek = function seek(key) {
 };
 
 Iterator.prototype.end = function end(callback) {
+  if (this._end)
+    return callback(new Error('end() already called on iterator.'));
   this._end = true;
   this.iter.end(callback);
 };