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.

ethcore/src/verification/queue/kind.rs

@@ -119,14 +119,13 @@ pub mod blocks {
 
 	impl Unverified {
 		/// Create an `Unverified` from raw bytes.
-		pub fn new(bytes: Bytes) -> Self {
-			use views::BlockView;
+		pub fn from_rlp(bytes: Bytes) -> Result<Self, ::rlp::DecoderError> {
 
-			let header = view!(BlockView, &bytes).header();
-			Unverified {
+			let header = ::rlp::Rlp::new(&bytes).val_at(0)?; 
+			Ok(Unverified {
 				header: header,
 				bytes: bytes,
-			}
+			})
 		}
 	}