stricter check for context bytes when recv LC data (#4480)

Bellatrix and Altair light client data share same body, but have other
fork digests. Validate that the peer's sent fork digest matches the one
that we expect (derived from `attested_header.slot`).

beacon_chain/sync/sync_protocol.nim

@@ -135,6 +135,8 @@ proc readChunkPayload*(
       return neterr InvalidContextBytes
   if res.isErr:
     return err(res.error)
+  if stateFork != peer.network.cfg.stateForkAtEpoch(res.get.contextEpoch):
+    return neterr InvalidContextBytes
   return ok res.get
 
 func shortLog*(s: StatusMsg): auto =