consensus/ethash: fix usage of *reflect.SliceHeader (#21372)

* consensus/ethash: only use *reflect.SliceHeader, not reflect.SliceHeader. See comment here: https://github.com/golang/go/issues/40397\#issuecomment-663748689



* consensus/ethash: pr feedback from @mdempsky, makes a copy of dest such that is not mutated

* consensus/ethash: remove noop assign

* consensus/ethash: apply same fix to another location

Co-authored-by: Péter Szilágyi <peterke@gmail.com>
Co-authored-by: Martin Holst Swende <martin@swende.se>