Minor fix in chain supplier and light provider (#8906)

* fix chain supplier increment

* fix light provider block_headers

ethcore/light/src/provider.rs

@@ -54,6 +54,7 @@ pub trait Provider: Send + Sync {
 	/// results within must adhere to the `skip` and `reverse` parameters.
 	fn block_headers(&self, req: request::CompleteHeadersRequest) -> Option<request::HeadersResponse> {
 		use request::HashOrNumber;
+		const MAX_HEADERS_TO_SEND: u64 = 512;
 
 		if req.max == 0 { return None }
 
@@ -82,10 +83,12 @@ pub trait Provider: Send + Sync {
 			}
 		};
 
-		let headers: Vec<_> = (0u64..req.max as u64)
-			.map(|x: u64| x.saturating_mul(req.skip + 1))
+		let max = ::std::cmp::min(MAX_HEADERS_TO_SEND, req.max);
+
+		let headers: Vec<_> = (0u64..max)
+			.map(|x: u64| x.saturating_mul(req.skip.saturating_add(1)))
 			.take_while(|x| if req.reverse { x < &start_num } else { best_num.saturating_sub(start_num) >= *x })
-			.map(|x| if req.reverse { start_num - x } else { start_num + x })
+			.map(|x| if req.reverse { start_num.saturating_sub(x) } else { start_num.saturating_add(x) })
 			.map(|x| self.block_header(BlockId::Number(x)))
 			.take_while(|x| x.is_some())
 			.flat_map(|x| x)

ethcore/sync/src/chain/supplier.rs

@@ -133,7 +133,7 @@ impl SyncSupplier {
 		let max_count = cmp::min(MAX_HEADERS_TO_SEND, max_headers);
 		let mut count = 0;
 		let mut data = Bytes::new();
-		let inc = (skip + 1) as BlockNumber;
+		let inc = skip.saturating_add(1) as BlockNumber;
 		let overlay = io.chain_overlay().read();
 
 		// We are checking the `overlay` as well since it's where the ForkBlock
@@ -155,9 +155,9 @@ impl SyncSupplier {
 				if number <= inc || number == 0 {
 					break;
 				}
-				number -= inc;
+				number = number.saturating_sub(inc);
 			} else {
-				number += inc;
+				number = number.saturating_add(inc);
 			}
 		}
 		let mut rlp = RlpStream::new_list(count as usize);