mpdparser: Be consistent about returning duplicated URL

mpdparser: Be consistent about returning duplicated URL

Instead of returning a "const gchar" or a "gchar" that should not be freed, always
return a duplicated string as those functions were used together with g_strdup anyway.

This is needed to prepare support for returning modified strings in next commit.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1147>