Add missing cs_main lock to ::GETBLOCKTXN processing

Note that this is not a major issue as, in order for the missing
lock to cause issues, you have to receive a GETBLOCKTXN message
while reindexing, adding a block header via RPC, etc, which results
in either a table rehash or an insert into the bucket which you are
currently looking at.

src/main.cpp

@@ -5442,6 +5442,8 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         BlockTransactionsRequest req;
         vRecv >> req;
 
+        LOCK(cs_main);
+
         BlockMap::iterator it = mapBlockIndex.find(req.blockhash);
         if (it == mapBlockIndex.end() || !(it->second->nStatus & BLOCK_HAVE_DATA)) {
             LogPrintf("Peer %d sent us a getblocktxn for a block we don't have", pfrom->id);