Set _filterstr in HintManager

This is also fixed in
fd12c7567e

But we need the fix in to make tests run.

qutebrowser/browser/hints.py

@@ -157,6 +157,7 @@ class HintManager(QObject):
         self._win_id = win_id
         self._tab_id = tab_id
         self._context = None
+        self._filterstr = None
         self._word_hinter = WordHinter()
         mode_manager = objreg.get('mode-manager', scope='window',
                                   window=win_id)