Check transaction count early in submitblock.

There is no point in even hashing a submitted block which doesn't have
 a coinbase transaction.

This also results in more useful error reporting on corrupted input.

Thanks to rawodb for the bug report.

src/rpc/mining.cpp

@@ -744,6 +744,10 @@ UniValue submitblock(const JSONRPCRequest& request)
     if (!DecodeHexBlk(block, request.params[0].get_str()))
         throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "Block decode failed");
 
+    if (block.vtx.empty() || !block.vtx[0]->IsCoinBase()) {
+        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "Block does not start with a coinbase");
+    }
+
     uint256 hash = block.GetHash();
     bool fBlockPresent = false;
     {