Speculative fix for permission crash

In some scenarios (which I can't seem to reproduce), the URL we get in
shared.feature_permission is an invalid one.

In that case, we set "urlstr = None", but that means we call message.* with
url=None but option=... set. Since that's invalid usage (we can't set a
per-domain option without knowing the URL) it bails out, causing a crash.

qutebrowser/browser/shared.py

@@ -221,6 +221,7 @@ def feature_permission(url, option, msg, yes_action, no_action, abort_on,
                 html.escape(url.toDisplayString()), msg)
         else:
             urlstr = None
+            option = None  # For message.ask/confirm_async
             text = "Allow the website to {}?".format(msg)
 
         if blocking: