Fix a bug in history command

This is a bug that was introduced recently when the functionality of the list command was merged with the history command.

This prevents an unhandled exception crash under certain unlikely but possible circumstances.

Author: tleonhardt

cmd2.py

@@ -1610,8 +1610,11 @@ def do_history(self, arg, opts):
         if arg:
             # If a character indicating a slice is present, retrieve a slice of the history
             if '..' in arg or ':' in arg:
-                # Get a slice of history
-                history = self.history.span(arg)
+                try:
+                    # Get a slice of history
+                    history = self.history.span(arg)
+                except IndexError:
+                    history = self.history.get(arg)
             else:
                 # Get item(s) from history by index or string search
                 history = self.history.get(arg)