Fix getblocktemplate RPC returning wrong reserved_offset

The byte for TX_EXTRA_TAG_PUBKEY is already included in
res.reserved_offset by the call to slow_memmem.

src/rpc/core_rpc_server.cpp

@@ -945,7 +945,7 @@ namespace cryptonote
       LOG_ERROR("Failed to find tx pub key in blockblob");
       return false;
     }
-    res.reserved_offset += sizeof(tx_pub_key) + 3; //3 bytes: tag for TX_EXTRA_TAG_PUBKEY(1 byte), tag for TX_EXTRA_NONCE(1 byte), counter in TX_EXTRA_NONCE(1 byte)
+    res.reserved_offset += sizeof(tx_pub_key) + 2; //2 bytes: tag for TX_EXTRA_NONCE(1 byte), counter in TX_EXTRA_NONCE(1 byte)
     if(res.reserved_offset + req.reserve_size > block_blob.size())
     {
       error_resp.code = CORE_RPC_ERROR_CODE_INTERNAL_ERROR;