8276e70de Adding assert to avoid a memory access violation inside of PartialMerkleTree::CalcHash() (Chris Stewart)
Pull request description:
Fixing a possible memory access violation in CPartialMerkleTree::CalcHash(). This can happen if we some how a merkle tree with zero txids. I don't think this can happen in practice as we only send merkle block messages on the p2p network as of now -- we cannot receive them. This was found with #8469, specifically using this [generator](https://github.com/Christewart/bitcoin/blob/rapidcheck/src/test/gen/merkleblock_gen.h#L52-L77) which will cause a memory access violation on [this test case](https://github.com/Christewart/bitcoin/blob/rapidcheck/src/test/merkleblock_properties.cpp#L48).
Backport of Core PR9980
Reviewer note: This seems like a vulnerability waiting to happen, so I carefully looked over it's usage. Short summary:
- It cannot be exploited at present.
- The node does not handle receiving CMerkleBlock, it only contructs and sends them from (and this is the important part) blocks that have been validated.
- Potential future vectors to attack this assert could include:
- Invalid block reflection attack if net_processing ever stores an invalid, 0-tx block in most_recent_block
- Any new code that accepts CMerkleBlock from peers.