Fix mem access violation merkleblock

src/merkleblock.cpp

@@ -59,6 +59,9 @@ 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
 {