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

@@ -248,9 +248,10 @@ 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 0;
+}
 
 static uint8_t char2int(char c) {
     if (c >= '0' && c <= '9') return c - '0';