Don't fully clean up labels if their frame is gone

Otherwise they're invalid but still in the HintContext, so calling
.hide() on it later (e.g. because the user pressed another key) would
give us a RuntimeError from PyQt.

qutebrowser/browser/hints.py

@@ -121,7 +121,7 @@ class HintLabel(QLabel):
         log.hints.debug("Contents size changed...!")
         if self.elem.frame() is None:
             # This sometimes happens for some reason...
-            self.cleanup()
+            self.hide()
         else:
             self._move_to_elem()