It is better to be safe than to do anything that may place us in dangerous position. Here ‘sure’ is used in a old sense of ‘safe’, ‘free from danger’. ‘If we reach the forest’, says in a character of Shakespeare, ‘we shall be sure enough’.
Two boys were chased across a field by a bull. They just managed the escape its horn by climbing up a tree. After half an hour or so one said:
‘We can’t stop here all the day. Let’s make a run for it.’
‘You do as you like,’ said the other. ‘I’m stopping where I am till a farmer comes. Better be sure than sorry.’