Merge #9980: Fix mem access violation merkleblock

Merge #9980: Fix mem access violation merkleblock

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).

Tree-SHA512: b95904ec45ea3f082c7722161d93ee06b24c706fbffa909a6e995ed14788aed2830f91b626da6f0347660c45874a0735dab61c9440b59c949c690af4165c83fb