Folder.getlast(): avoid PyChecker warning.

Lib/mhlib.py

@@ -648,7 +648,7 @@ class Folder:
     def getlast(self):
         """Return the last message number."""
         if not hasattr(self, 'last'):
-            messages = self.listmessages()
+            self.listmessages() # Set self.last
         return self.last
 
     def setlast(self, last):