src: Replace implicit conversion of st::string to ndn::Name

Fixed two places where std::string was passed to function accepting ndn::Name.
Although no compilation error was shown, from NLSR log investigation it is
found that name is garbled in CentOs because of that string passing.

Refs: #1713

Change-Id: I8a9fc00144df383debaac1e1c418fca14617f113