node limit passed as 0 instead of a negative number to visits_stopper if nn cache > memory limit (prevents unsigned int variable overflow)

[KarlKfoury]

if cache size > ram limit, passing (ram limit) - (cache size) to visitsstopper as argument for node limit, which is an unsigned int, results in an error. Which is why it is necessary to pass 0 instead of (ram limit) - (cache size) if cache size > ram limit. Adressing issue #1597