wallet: fix boost::get usage with boost 1.58
wallet: fix boost::get usage with boost 1.58

Lifted from https://github.com/bitcoin/bitcoin/pull/6114/files

Quoins1/Quoins,Quoins1/Quoins,Quoins1/Quoins,Quoins1/Quoins,Quoins1/Quoins