VM: better logic in factor_vm::set_sampling_profiler
VM: better logic in factor_vm::set_sampling_profiler

It avoids the double-negation.

factor/factor,bjourne/factor,bjourne/factor,bjourne/factor,AlexIljin/factor,bjourne/factor,AlexIljin/factor,bpollack/factor,bjourne/factor,bpollack/factor,AlexIljin/factor,tgunr/factor,tgunr/factor,mrjbq7/factor,factor/factor,factor/factor,mrjbq7/factor,mrjbq7/factor,bpollack/factor,tgunr/factor,AlexIljin/factor,AlexIljin/factor,bpollack/factor,tgunr/factor,bjourne/factor,bpollack/factor,AlexIljin/factor,mrjbq7/factor,bjourne/factor,mrjbq7/factor,factor/factor,factor/factor,factor/factor,AlexIljin/factor,mrjbq7/factor,bpollack/factor,tgunr/factor,bpollack/factor,tgunr/factor