message: avoid overflow in storage component
diff: @@ -1890,7 +1890,7 @@ impl Storage {
  ) -> Result<Option<BlockSignatures>, FatalStorageError> {
  if let Some(last_emergency_restart) = self.last_emergency_restart {
  if block_header.era_id() <= last_emergency_restart {
  - debug!(
  + warn!(
  ?block_header,
  ?last_emergency_restart,
  "finality signatures from before last emergency restart requested"
  @@ -1905,9 +1905,11 @@ impl Storage {
  None => return Ok(None),
  Some(block_signatures) => block_signatures,
  };
  + // If `block_header` is from era 0, we can use the switch block from era 0 to ascertain the
  + // validators for that era.
  let switch_block_hash = match self
  .switch_block_era_id_index
  - .get(&(block_header.era_id() - 1))
  + .get(&(block_header.era_id().saturating_sub(1)))
  {
  None => return Ok(None),
  Some(switch_block_hash) => switch_block_hash,
  