xspf muxer: %encode '+' since decode_URI() will decode it as a space

modules/misc/playlist/xspf.c

@@ -336,6 +336,7 @@ static char *assertUTF8URI( char *psz_name )
             *psz_s == '[' ||
             *psz_s == ']' ||
             *psz_s == '@' ||
+            *psz_s == '+' ||
             *psz_s == '%' )
             )
           )