Don't allocate in expect_valid_rlp unless necessary (#8867)

* don't allocate via format! in case there's no error

* fix test?

ethcore/src/views/view_rlp.rs

@@ -53,7 +53,12 @@ impl<'a, 'view> ViewRlp<'a> where 'a : 'view {
 	}
 
 	fn expect_valid_rlp<T>(&self, r: Result<T, DecoderError>) -> T {
-		r.expect(&format!("View rlp is trusted and should be valid. Constructed in {} on line {}", self.file, self.line))
+		r.unwrap_or_else(|e| panic!(
+			"View rlp is trusted and should be valid. Constructed in {} on line {}: {}",
+			self.file,
+			self.line,
+			e
+		))
 	}
 
 	/// Returns rlp at the given index, panics if no rlp at that index