leveled: fix a bug may cause panic

[WangXiangUSTC]

fix issue: #224

[tgulacsi]

Calling with slice.Start > slice.Limit is a programmer's error, and should panic.
Please add an explicit

if slice.Start != nil && slice.Limit != nil && *slice.Start > *slice.Limit { 
    panic("slice.Start cannot be greater than slice.Limit") 
}

right after the slice != nil check, to keep the panic when the caller provided bogus Range.