spv: Validate header chain diff earlier

This moves the header chain difficulty validation to an earlier point in
time of the initial sync process.

This is preferable so that less work is performed in case invalid
headers are received.

spv/sync.go

@@ -1338,6 +1338,22 @@ func (s *Syncer) getHeaders(ctx context.Context, rp *p2p.RemotePeer) error {
 			}
 		}
 
+		// Verify the sidechain that includes the received headers has
+		// the correct difficulty.
+		s.sidechainMu.Lock()
+		fullsc, err := s.sidechains.FullSideChain(nodes)
+		if err != nil {
+			s.sidechainMu.Unlock()
+			return err
+		}
+		_, err = s.wallet.ValidateHeaderChainDifficulties(ctx, fullsc, 0)
+		if err != nil {
+			s.sidechainMu.Unlock()
+			rp.Disconnect(err)
+			return err
+		}
+		s.sidechainMu.Unlock()
+
 		g, ctx := errgroup.WithContext(ctx)
 		for i := range headers {
 			i := i
@@ -1408,12 +1424,6 @@ func (s *Syncer) getHeaders(ctx context.Context, rp *p2p.RemotePeer) error {
 			continue
 		}
 
-		_, err = s.wallet.ValidateHeaderChainDifficulties(ctx, bestChain, 0)
-		if err != nil {
-			s.sidechainMu.Unlock()
-			return err
-		}
-
 		prevChain, err := s.wallet.ChainSwitch(ctx, &s.sidechains, bestChain, nil)
 		if err != nil {
 			s.sidechainMu.Unlock()