trie, eth/protocols/snap: sanitize the committed node data

[rjl493456442]

This pull request adds one more check before committing the trie nodes from the trie.Syncer:

• for node deletion, the node blob must be zero-length
• for node update, the node blob must be non-zero-length

The check is cheap enough and shouldn't have any performance impact.

---

The node blob cached in the trie.Syncer is not deep-copied, rather than a reference of the network packet.

Theoretically it should be safe the hold the network packet reference being originated from here, although i am not 100% confident about it.

If it's possible to empties the referenced blob somehow, than it could be the root cause of #29436

[holiman]

Shouldn't you rewrite isDelete into:

func (op *nodeOp) isDelete() bool {
	return op.del
}

Alternatively: delete the method

[rjl493456442]

Yep, the method should be deleted

[karalabe]

This PR seems simple and good, just maybe not needed (making the surrounding code harder to understand). My 2c, lets leave it open until the root cause for the sync issue is discovered and if it's related to this, we should merge. If it's unrelated, it might be better to close.

[holiman]

LGTM