fixed wav riff tag size

Originally committed as revision 95 to svn://svn.ffmpeg.org/ffmpeg/trunk

libav/wav.c

@@ -72,7 +72,7 @@ static int wav_write_trailer(AVFormatContext *s)
         /* update file size */
         file_size = url_ftell(pb);
         url_fseek(pb, 4, SEEK_SET);
-        put_le32(pb, (UINT32)file_size);
+        put_le32(pb, (UINT32)(file_size - 8));
         url_fseek(pb, file_size, SEEK_SET);
 
         put_flush_packet(pb);