avformat/wavdec: Dont trust the fact chunk for PCM

Fixes Ticket3033
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/wavdec.c

@@ -403,11 +403,15 @@ break_loop:
 
     avio_seek(pb, data_ofs, SEEK_SET);
 
-    if (!sample_count && st->codec->channels &&
-        av_get_bits_per_sample(st->codec->codec_id) && wav->data_end <= avio_size(pb))
-        sample_count = (data_size << 3) /
-                       (st->codec->channels *
-                        (uint64_t)av_get_bits_per_sample(st->codec->codec_id));
+    if (!sample_count || av_get_exact_bits_per_sample(st->codec->codec_id) > 0)
+        if (   st->codec->channels
+            && data_size
+            && av_get_bits_per_sample(st->codec->codec_id)
+            && wav->data_end <= avio_size(pb))
+            sample_count = (data_size << 3)
+                                  /
+                (st->codec->channels * (uint64_t)av_get_bits_per_sample(st->codec->codec_id));
+
     if (sample_count)
         st->duration = sample_count;