nodes panic when checking incoming blocks

[DrZoltanFazekas]

We observed this on devnet on several API nodes. The log says:

message: "called `Option::unwrap()` on a `None` value"
panic.column: 68
panic.file: "zilliqa/src/consensus.rs"
panic.line: 1731

It was the unwrap in consensus.rs on line 1731:

        // Derive the proposer from the block's view
        let proposer = self.leader_at_block(&parent, block.view()).unwrap();

since leader_at_block_raw() returned None on line 2115-2117 because the parent block was missing:

        let Ok(state_at) = self.try_get_state_at(block.number()) else {
            return None;
        };

which is strange because we would have returned an error on line 1710 before we even attempt to get the proposer on line 1731:

        let Some(parent) = self
            .get_block(&block.parent_hash())
            .map_err(|e| (e, false))?
        else {
            warn!(
                "Missing parent block while trying to check validity of block {}",
                block.number()
            );
            return Err((MissingBlockError::from(block.parent_hash()).into(), true));
        };

[JamesHinshelwood]

Caused by #1056