Don't panic in import_block if invalid rlp (#8522)

* Don't panic in import_block if invalid rlp

* Remove redundant type annotation

* Replace RLP header view usage with safe decoding

Using the view will panic with invalid RLP. Here we use Rlp decoding directly which will return a `Result<_, DecoderError>`. While this path currently should not have any invalid RLP - it makes it safer if ever called with invalid RLP from other code paths.