Fix download index in downloadFinished

An index of 0 is valid as this is a bog standard array.  This resolves an instance of the warning "no download index 0".

src/downloadmanager.cpp

@@ -1655,7 +1655,7 @@ void DownloadManager::nxmRequestFailed(QString gameName, int modID, int fileID,
 void DownloadManager::downloadFinished(int index)
 {
   DownloadInfo *info;
-  if (index)
+  if (index >= 0)
     info = m_ActiveDownloads[index];
   else
     info = findDownload(this->sender(), &index);