avformat/oggenc: ignore empty packets

Some encoders, like flac, can send side data only packets at the end.
Eventually, said extradata update should ideally be used to update the header
when writting to seekable output, but for now, ignore them.

Should fix the undefined behavior of passing NULL to memcpy().

Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/oggenc.c

@@ -687,7 +687,7 @@ static int ogg_write_packet(AVFormatContext *s, AVPacket *pkt)
 {
     int i;
 
-    if (pkt)
+    if (pkt && pkt->size)
         return ogg_write_packet_internal(s, pkt);
 
     for (i = 0; i < s->nb_streams; i++) {

tests/ref/lavf/ogg

@@ -1,3 +1,3 @@
-81b9366cacb23644c2803585dced9996 *tests/data/lavf/lavf.ogg
+507a906a705d16f3a3b0c4114c738110 *tests/data/lavf/lavf.ogg
 13516 tests/data/lavf/lavf.ogg
 tests/data/lavf/lavf.ogg CRC=0x3a1da17e