Merge bitcoin#9980: Fix mem access violation merkleblock

8276e70 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 bitcoin#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

src/merkleblock.cpp

@@ -71,6 +71,9 @@ CMerkleBlock::CMerkleBlock(const CBlock& block, const std::set<uint256>& txids)
 }
 
 uint256 CPartialMerkleTree::CalcHash(int height, unsigned int pos, const std::vector<uint256> &vTxid) {
+    //we can never have zero txs in a merkle block, we always need the coinbase tx
+    //if we do not have this assert, we can hit a memory access violation when indexing into vTxid
+    assert(vTxid.size() != 0);
     if (height == 0) {
         // hash at height 0 is the txids themself
         return vTxid[pos];

src/merkleblock.h

@@ -121,6 +121,8 @@ class CPartialMerkleTree
 /**
  * Used to relay blocks as header + vector<merkle branch>
  * to filtered nodes.
+ *
+ * NOTE: The class assumes that the given CBlock has *at least* 1 transaction. If the CBlock has 0 txs, it will hit an assertion.
  */
 class CMerkleBlock
 {