Fix non-const mapMultiArgs[] access after init.

Swap mapMultiArgs for a const-reference to a _mapMultiArgs which is
only accessed in util.cpp

src/httprpc.cpp

@@ -95,7 +95,7 @@ static bool multiUserAuthorized(std::string strUserPass)
 
     if (mapMultiArgs.count("-rpcauth") > 0) {
         //Search for multi-user login/pass "rpcauth" from config
-        BOOST_FOREACH(std::string strRPCAuth, mapMultiArgs["-rpcauth"])
+        BOOST_FOREACH(std::string strRPCAuth, mapMultiArgs.at("-rpcauth"))
         {
             std::vector<std::string> vFields;
             boost::split(vFields, strRPCAuth, boost::is_any_of(":$"));