libavformat/utils: Warning about invalid cover art instead of an error

This way other streams can still be used with the -map option.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -517,8 +517,12 @@ int avformat_queue_attached_pictures(AVFormatContext *s)
         if (s->streams[i]->disposition & AV_DISPOSITION_ATTACHED_PIC &&
             s->streams[i]->discard < AVDISCARD_ALL) {
             AVPacket copy = s->streams[i]->attached_pic;
-            if (copy.size <= 0)
-                return AVERROR(EINVAL);
+            if (copy.size <= 0) {
+                av_log(s, AV_LOG_WARNING,
+                    "Attached picture on stream %d has invalid size, "
+                    "ignoring\n", i);
+                continue;
+            }
             copy.buf = av_buffer_ref(copy.buf);
             if (!copy.buf)
                 return AVERROR(ENOMEM);