It is a play on words form a more-normal figurative expression. . "Do not count your chickens before they are hatched" means do not assume everything will go well. Do not assume things will be 100% successful. Yes, a farming analogue. It is a warning not to be too optimistic. . Your version. "Don't un-count your chickens until you know they are dead." Says do not assume the worst until it is proven. Things may not be bad. For example something is not going weel, but it may yet recover. It is a suggestion to not be too pessimistic.

From now on, I'm going to use this version!

“Don’t count your chickens ...” is well-known and standard. “Don’t uncount you chickens ...” isn’t known or standard.