Don't try to modify buffer line when message not found
Don't try to modify buffer line when message not found

If the timestamp given to change_message doesn't exist in the messages
dict, just return.

Previously an unknown ts would make modify_buffer_line crash unless text
was set, because modify_buffer_line fails if text is None.

trygveaa/wee-slack,wee-slack/wee-slack,rawdigits/wee-slack