You're overflowing the Int datatype, because the result of fib(93) (12,200,160,415,121,876,738, roughly 1.22 x 10^19) is larger than Int64.max (9,223,372,036,854,775,807, or roughly 9.22 x 10^18).
On 32 bit machines, this will crash even earlier, at fib(47) (2,971,215,073), because Int is a 32 bit type on 32 bit machines. Int32.max is only 2,147,483,647.
