demux_lavf: fix check for files lavf doesn't recognize

Commit 91ea30c585 ("demux_lavf: use lavf for all formats except those
listed") broke handling of files whose type libavformat couldn't
recognize at all. Fix the demux_lavf probe function to correctly
return failure in that case.

libmpdemux/demux_lavf.c

@@ -248,8 +248,9 @@ static int lavf_check_preferred_file(demuxer_t *demuxer){
         for (p = preferred_internal; *p; p++)
             if (matches_avinputformat_name(priv, *p))
                 return 0;
+        return DEMUXER_TYPE_LAVF_PREFERRED;
     }
-    return DEMUXER_TYPE_LAVF_PREFERRED;
+    return 0;
 }
 
 static uint8_t char2int(char c) {