fix a nil pointer crash due to previous revert

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

core/block_validator.go

@@ -125,7 +125,10 @@ 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 {
-	return v.engine.VerifyHeader(v.bc, block.Header(), true)
+	if h := block.Header(); h != nil {
+		return v.engine.VerifyHeader(v.bc, h, true)
+	}
+	return errors.New("header field was nil")
 }
 
 // ValidateHeaders verifies a batch of blocks' headers concurrently. The method returns a quit channel