xspf: Fix a crash on track elements without a location

A <track> can have 0 or more <location> elements. VLC doesn't like elements without a uri however, so init an element with vlc://nop. Note also that we currently only support 1 location element per track....

This closes #2891

modules/demux/playlist/xspf.c

@@ -416,7 +416,7 @@ static bool parse_track_node COMPLEX_INTERFACE
           {NULL,           UNKNOWN_CONTENT, {NULL} }
         };
 
-    input_item_t *p_new_input = input_item_New( p_demux, NULL, NULL );
+    input_item_t *p_new_input = input_item_New( p_demux, "vlc://nop", NULL );
 
     if( !p_new_input )
     {