core: BlockFetcher.(GetBlock, GetBlockByHeight) are susceptible to a panic due to insufficient nil checks

[odeke-em]

If we examine this code

celestia-node/core/fetcher.go

Lines 76 to 81 in 896a46b

	if res != nil && res.Block == nil {
	return nil, fmt.Errorf("core/fetcher: block not found, hash: %s", hash.String())
	}
	return res.Block, nil
	}

we notice that the first condition checks if res != nil and then if res.Block == nil. However by the time that we are checking if res != nil it can really be nil, yet on line 81 we are nakedly dereferncing res.Block

Suggestion

That code could be made much more whole and include a check for if res == nil || res.Block == nil { and then also add a test.

/cc @Wondertan @liamsi @cristaloleg

[Wondertan]

That's true, although in practise this case is not happening. Generally we think this result based api is confusing and error prone.

Care for a PR?