Adding assert to avoid a memory access violation inside of PartialMerkleTree::CalcHash()

Adding comment to assert in PartialMerkleTree::CalcHash()

Adding comment on CMerkleBlock indicating it calls something that contains an assert

Removing EOL whitespace

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];