[meld] undo: Make our can undo/redo checks more defensive

[Kai Willadsen <kaiw src gnome org>]

commit d4ac9b4f34f3e52c0984b3b4f2ef99b85ca61298
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Sun Feb 11 08:14:50 2018 +1000

    undo: Make our can undo/redo checks more defensive
    
    This looks pointless, but it's about to be useful, I promise.

 meld/undo.py | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)
---
diff --git a/meld/undo.py b/meld/undo.py
index 8502eff3..fa634ff3 100644
--- a/meld/undo.py
+++ b/meld/undo.py
@@ -100,14 +100,13 @@ class UndoSequence(GObject.GObject):
         self.checkpoints = {}
 
     def can_undo(self):
-        """Return if an undo is possible.
-        """
-        return self.next_redo > 0
+        """Return whether an undo is possible."""
+        return getattr(self, 'next_redo', 0) > 0
 
     def can_redo(self):
-        """Return if a redo is possible.
-        """
-        return self.next_redo < len(self.actions)
+        """Return whether a redo is possible."""
+        next_redo = getattr(self, 'next_redo', 0)
+        return next_redo < len(getattr(self, 'actions', []))
 
     def add_action(self, action):
         """Add an action to the undo list.