Handle invalid QUrl objects in HostBlocker.is_blocked

For some requests, we don't know a first party URL, so we get an invalid QUrl
object from QtWebKit/QtWebEngine.

However, the config system wants either a valid QUrl or None (for the global
value), so we don't accidentally pass an invalid QUrl in. Thus, do the
conversion here.

qutebrowser/browser/adblock.py

@@ -112,9 +112,12 @@ class HostBlocker:
 
     def is_blocked(self, url, first_party_url=None):
         """Check if the given URL (as QUrl) is blocked."""
+        if first_party_url is not None and not first_party_url.isValid():
+            first_party_url = None
         if not config.instance.get('content.host_blocking.enabled',
                                    url=first_party_url):
             return False
+
         host = url.host()
         return ((host in self._blocked_hosts or
                  host in self._config_blocked_hosts) and