storage/curl: fix nullptr dereference

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

src/storage/plugins/CurlStorage.cxx

@@ -80,7 +80,7 @@ std::string_view
 CurlStorage::MapToRelativeUTF8(std::string_view uri_utf8) const noexcept
 {
 	return PathTraitsUTF8::Relative(base,
-					CurlUnescape(uri_utf8).c_str());
+					CurlUnescape(uri_utf8));
 }
 
 class BlockingHttpRequest : protected CurlResponseHandler {