mruby-time: add check to prevent invalid time

```ruby
Time.at(-1)  # => ïD$♦ ░`☻ Sat -1 -1:-1:-1 1899
Time.at(100) - 101  # => ïD$♦ ░`☻ Sat -1 -1:-1:-1 1899
```
I added a check to mrb_time_make that'll raise an ArgumentError instead.