fix underflow in pip, closes #10419 (#10423)

ethcore/light/src/net/mod.rs

@@ -533,6 +533,9 @@ impl LightProtocol {
 			// the timer approach will skip 1 (possibly 2) in rare occasions.
 			if peer_info.sent_head == announcement.head_hash ||
 				peer_info.status.head_num >= announcement.head_num ||
+				// fix for underflow reported in
+				// https://github.com/paritytech/parity-ethereum/issues/10419
+				now < peer_info.last_update ||
 				now - peer_info.last_update < UPDATE_INTERVAL {
 				continue
 			}