Don't crash if data is None while saving session

Under some circumstances I can't reproduce (switching/turning off
monitors?) it seems it's possible that SessionManager.save gets called
with last_window=True, without on_last_window_closed being called.

This might be to one of the Qt screen management bugs fixed in Qt 5.6,
which would explain why I can't reproduce it.

Instead of crashing, let's log the error and not save the session.

qutebrowser/misc/sessions.py

@@ -241,7 +241,9 @@ class SessionManager(QObject):
         log.sessions.debug("Saving session {} to {}...".format(name, path))
         if last_window:
             data = self._last_window_session
-            assert data is not None
+            if data is None:
+                log.sessions.error("last_window_session is None while saving!")
+                return
         else:
             data = self._save_all()
         log.sessions.vdebug("Saving data: {}".format(data))