Ignore deleted windows when saving session.

This hopefully helps with a crash on Windows which happens from time to time
during some multi-window tabs.feature tests:

Traceback (most recent call last):
  File "...\mainwindow\mainwindow.py", line 446, in closeEvent
    self._do_close()
  File "...\mainwindow\mainwindow.py", line 405, in _do_close
    objreg.get('session-manager').save_last_window_session()
  File "...\misc\sessions.py", line 247, in save_last_window_session
    self._last_window_session = self._save_all()
  File "...\misc\sessions.py", line 183, in _save_all
    win_data['geometry'] = bytes(main_window.saveGeometry())
RuntimeError: wrapped C/C++ object of type MainWindow has been deleted

qutebrowser/misc/sessions.py

@@ -20,6 +20,7 @@
 """Management of sessions - saved tabs/windows."""
 
 import os
+import sip
 import os.path
 
 from PyQt5.QtCore import pyqtSignal, QUrl, QObject, QPoint, QTimer
@@ -176,6 +177,11 @@ class SessionManager(QObject):
                                         window=win_id)
             main_window = objreg.get('main-window', scope='window',
                                      window=win_id)
+
+            # We could be in the middle of destroying a window here
+            if sip.isdeleted(main_window):
+                continue
+
             win_data = {}
             active_window = QApplication.instance().activeWindow()
             if getattr(active_window, 'win_id', None) == win_id: