message: Fix error on older gcc
  On Ubuntu 16.04 with gcc 5.4.0 the code generated this error:
  static assertion failed: boost::variant does not contain specified type U, call to boost::get<U>(boost::variant<T...>&) will always throw boost::bad_get exception
diff: @@ -239,7 +239,7 @@ Value listunspent(const Array& params, bool fHelp)
  CTxDestination address;
  if (ExtractDestination(pk, address))
  {
  - const CScriptID& hash = boost::get<const CScriptID&>(address);
  + const CScriptID& hash = boost::get<CScriptID>(address);
  CScript redeemScript;
  if (pwalletMain->GetCScript(hash, redeemScript))
  entry.push_back(Pair("redeemScript", HexStr(redeemScript.begin(), redeemScript.end())));
  