grandpa: fix warp sync proof on missing data (#8795)

* grandpa: check for missing data when iterating through authority set changes

* grandpa-warp-sync: handle missing data

substrate/client/finality-grandpa-warp-sync/src/lib.rs

@@ -173,4 +173,6 @@ pub enum HandleRequestError {
 	InvalidProof(String),
 	#[display(fmt = "Failed to send response.")]
 	SendResponse,
+	#[display(fmt = "Missing required data to be able to answer request.")]
+	MissingData,
 }

substrate/client/finality-grandpa-warp-sync/src/proof.rs

@@ -91,7 +91,10 @@ impl<Block: BlockT> WarpSyncProof<Block> {
 		let mut proofs_encoded_len = 0;
 		let mut proof_limit_reached = false;
 
-		for (_, last_block) in set_changes.iter_from(begin_number) {
+		let set_changes = set_changes.iter_from(begin_number)
+			.ok_or(HandleRequestError::MissingData)?;
+
+		for (_, last_block) in set_changes {
 			let header = blockchain.header(BlockId::Number(*last_block))?.expect(
 				"header number comes from previously applied set changes; must exist in db; qed.",
 			);