asf: don't seek back on EOF.

Seeking back on EOF will reset the EOF flag, causing us to re-enter
the loop to find the next marker in the ASF file, thus potentially
causing an infinite loop.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
(cherry picked from commit bb6d5411e1)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>

libavformat/asfdec.c

@@ -761,7 +761,7 @@ static int ff_asf_get_packet(AVFormatContext *s, AVIOContext *pb)
         c= avio_r8(pb);
         d= avio_r8(pb);
         rsize+=3;
-    }else{
+    } else if (!pb->eof_reached) {
         avio_seek(pb, -1, SEEK_CUR); //FIXME
     }