avformat/cdg: Do not fail if filesize cannot be determined

This fixes cdg with piped input which was broken by the previous commit
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/cdg.c

@@ -46,10 +46,11 @@ static int read_header(AVFormatContext *s)
     avpriv_set_pts_info(vst, 32, 1, 300);
 
     ret = avio_size(s->pb);
-    if (ret < 0)
-        return ret;
-
+    if (ret < 0) {
+        av_log(s, AV_LOG_WARNING, "Cannot calculate duration as file size cannot be determined\n");
+    } else
         vst->duration = (ret * vst->time_base.den) / (CDG_PACKET_SIZE * 300);
+
     return 0;
 }