fix logic issue in GetPublicKeys function

internal/chain/engine.go

@@ -214,14 +214,16 @@ func (e *engineImpl) VerifyHeaderWithSignature(chain engine.ChainReader, header
 // GetPublicKeys finds the public keys of the committee that signed the block header
 func GetPublicKeys(chain engine.ChainReader, header *block.Header, reCalculate bool) ([]*bls.PublicKey, error) {
 	var shardState shard.State
+	var err error
 	if reCalculate {
 		shardState = core.CalculateShardState(header.Epoch())
-	}
-	shardState, err := chain.ReadShardState(header.Epoch())
+	} else {
+		shardState, err = chain.ReadShardState(header.Epoch())
 		if err != nil {
 			return nil, ctxerror.New("failed to read shard state of epoch",
 				"epoch", header.Epoch().Uint64())
 		}
+	}
 
 	committee := shardState.FindCommitteeByID(header.ShardID())
 	if committee == nil {