demux/playlist/xspf: fix input-item uri creation
demux/playlist/xspf: fix input-item uri creation

The previous implementation inaccurately treated "p_sys->psz_base" as
if it was referring to whatever directory (uri-wise) that contains the
playlist we are currently processing.

Given that this is not the case, it would not handle relative paths
correctly.

These changes make sure that we use ProcessMRL to properly handle the
paths found within the playlist (the function handles both absolute
and relative ones).

fixes #17506

Signed-off-by: Thomas Guillem <5f50a84c1fa3bcff146405017f36aec1a10a9e38@gllm.fr>

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc