demux_playlist: fix m3u detection logic

Caused failure to detect .pls files, because they were misdetected as
m3u. The problem is that "forcing playlist files" and "forcing a
specific playlist format" are not really treated separate, and in both
cases p->force is set to true. This made m3u detect all files as m3u
if --playlist was used. So correctly check whether the file format is
actually being probed or not.

demux/demux_playlist.c

@@ -69,7 +69,7 @@ static bool pl_eof(struct pl_parser *p)
 static int parse_m3u(struct pl_parser *p)
 {
     bstr line = bstr_strip(pl_get_line(p));
-    if (!p->force && !bstr_equals0(line, "#EXTM3U"))
+    if (p->probing && !bstr_equals0(line, "#EXTM3U"))
         return -1;
     if (p->probing)
         return 0;