Check if item is a file when using directory queue

This fixes issues with the item count being wrong and whole directories
being passed on to the sender.

ssm/message_directory.py

@@ -110,7 +110,10 @@ def _get_messages(self, sort_by_mtime=False):
                 for (file_name, _mtime) in sorted(unsorted_messages,
                                                   key=lambda tup: tup[1]):
                     # Store the sorted file paths in a class element.
-                    sorted_messages.append(file_name)
+                    if os.path.isfile(file_name):
+                        sorted_messages.append(file_name)
+                    else:
+                        continue
 
                 # Return the sorted list.
                 return sorted_messages