avidec: Fix demuxing of non seekable avis with multiple RIFFs

This solution uses the size of the first RIFF if its valid
to check frame size validity when the filesize could not be determined.
Feedback is welcome, other ideas and improvments as well!
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/avidec.c

@@ -944,7 +944,7 @@ start_sync:
         n= get_stream_idx(d+2);
         av_dlog(s, "%X %X %X %X %X %X %X %X %"PRId64" %u %d\n",
                 d[0], d[1], d[2], d[3], d[4], d[5], d[6], d[7], i, size, n);
-        if(i + (uint64_t)size > avi->fsize || d[0] > 127)
+        if(i*(avi->io_fsize>0) + (uint64_t)size > avi->fsize || d[0] > 127)
             continue;
 
         //parse ix##