[libra-net] fix a bug of wrong slice start index
[libra-net] fix a bug of wrong slice start index

Closes: #3149
Approved by: davidiw

aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra