We need more validation for step

[peterqiu07]

cron/parser.go

Line 313 in bc59245

if step == 0 {

There is not enough validation for step
So when we use cron expression like "0/9999 * * * * ?", it still works, but it seems like out of expectation, and more strange
Further more, what do you expect for "0/9999 0/666 * * * ?"

Maybe we need to use the bounds to limit range as well as start and end