Qui va à la chasse perd sa place.
He who goes hunting loses his place.
Now, I've been racking my brains to think of a direct English equivalent. The closest I can get are 'look before you leap' and, indeed, the Alexander Pope quote 'fools rush in where angels fear to tread'. But these both are merely about the wisdom of being cautious in ones endeavours; I can't think of any that includes the specific connotation that going after something involves the risk that you could lose what you already have.
Do we have such a phrase?