Pass a std::string to PathTraitsUTF8::Relative(), implicitly casting
it to std::string_view. This selects the right overload which returns
std::string_view instead of `const char *`; the latter could return
`nullptr` which would cause the implicit conversion of the return
value to std::string_view to crash.
Regression caused by commits ead208987d and a98d627c0b.
Closes https://github.com/MusicPlayerDaemon/MPD/issues/995
f849b07766