Issue 4576 - Fixed a mistake introduced in previous commit

lib/io.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 79 matching lines @@
             let oldBuffer = buffer;
             buffer = data.substr(index + 1);
             data = data.substr(0, index + 1);
             let lines = data.split(/[\r\n]+/);
             lines.pop();
             lines[0] = oldBuffer + lines[0];
             for (let i = 0; i < lines.length; i++)
             {
               let promise = listener.process(lines[i]);
               if (promise)
-                yield listener.process(lines[i]);
+                yield promise;
             }
           }
           finally
           {
             processing = false;
             data = buffer;
             buffer = "";
             yield* onProgress(data);
 
             if (loaded)
@@ skipping 219 matching lines @@
         else
           callback(e);
       });
     }
     catch(e)
     {
       callback(e);
     }
   }
 }