avoid overwriting pkt

Originally committed as revision 6723 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/gxfenc.c

@@ -792,7 +792,7 @@ static int gxf_interleave_packet(AVFormatContext *s, AVPacket *out, AVPacket *pk
                 pkt = NULL;
             }
             if (flush || av_fifo_size(&sc->audio_buffer) >= GXF_AUDIO_PACKET_SIZE) {
-                if (gxf_new_audio_packet(gxf, sc, &new_pkt, flush) > 0) {
+                if (!pkt && gxf_new_audio_packet(gxf, sc, &new_pkt, flush) > 0) {
                     pkt = &new_pkt;
                     break; /* add pkt right now into list */
                 }