fix nil pointer of block

Signed-off-by: Leo Chen <leo@harmony.one>

core/block_validator.go

@@ -125,6 +125,9 @@ func (v *BlockValidator) ValidateState(block *types.Block, statedb *state.DB, re
 // given engine. Verifying the seal may be done optionally here, or explicitly
 // via the VerifySeal method.
 func (v *BlockValidator) ValidateHeader(block *types.Block, seal bool) error {
+	if block == nil {
+		return errors.New("block is nil")
+	}
 	if h := block.Header(); h != nil {
 		return v.engine.VerifyHeader(v.bc, h, true)
 	}