Fix nil signature verification

chain/types/blockheader.go

@@ -120,6 +120,10 @@ func (blk *BlockHeader) CheckBlockSignature(ctx context.Context, worker address.
 	_, span := trace.StartSpan(ctx, "checkBlockSignature")
 	defer span.End()
 
+	if blk.BlockSig == nil {
+		return xerrors.New("block signature not present")
+	}
+
 	sigb, err := blk.SigningBytes()
 	if err != nil {
 		return xerrors.Errorf("failed to get block signing bytes: %w", err)

chain/types/signature.go

@@ -116,5 +116,8 @@ func (s *Signature) UnmarshalCBOR(br io.Reader) error {
 }
 
 func (s *Signature) Equals(o *Signature) bool {
+	if s == nil || o == nil {
+		return s == o
+	}
 	return s.Type == o.Type && bytes.Equal(s.Data, o.Data)
 }

chain/types/signature_cgo.go

@@ -9,9 +9,14 @@ import (
 	"github.com/filecoin-project/go-address"
 	"github.com/filecoin-project/go-crypto"
 	"github.com/minio/blake2b-simd"
+	"golang.org/x/xerrors"
 )
 
 func (s *Signature) Verify(addr address.Address, msg []byte) error {
+	if s == nil {
+		return xerrors.Errorf("signature is nil")
+	}
+
 	if addr.Protocol() == address.ID {
 		return fmt.Errorf("must resolve ID addresses before using them to verify a signature")
 	}