wv: do not report invalid stream duration

If total number of samples is -1 it means stream is of unknown length.

Fixes ticket #1777.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavformat/wv.c

@@ -255,6 +255,7 @@ static int wv_read_header(AVFormatContext *s)
     st->codec->bits_per_coded_sample = wc->bpp;
     avpriv_set_pts_info(st, 64, 1, wc->rate);
     st->start_time = 0;
+    if (wc->samples != 0xFFFFFFFFu)
         st->duration = wc->samples;
 
     if (s->pb->seekable) {