Check for None-reply in _do_die

Fixes #2304

In some cases, the finished handler fired before the error handler, e.g.
when downloading a 500 error page that is sent as attachment:

    HTTP/1.1 500 Internal Server Error
    Content-Type: application/octet-stream
    Content-Disposition: inline; filename="attachment.jpg"

here we downloaded 0 bytes, fired the finished handler and after that
fired the error handler because of the 500 - but the finished handler
had already set our reply to None (and displayed the error message).

qutebrowser/browser/qtnetworkdownloads.py

@@ -110,6 +110,9 @@ class DownloadItem(downloads.AbstractDownloadItem):
     def _do_die(self):
         """Abort the download and emit an error."""
         self._read_timer.stop()
+        if self._reply is None:
+            log.downloads.debug("Reply gone while dying")
+            return
         self._reply.downloadProgress.disconnect()
         self._reply.finished.disconnect()
         self._reply.error.disconnect()