oggdec: fix invalid free on error

The read_packet callback passes a pointer to a stack-allocated AVPacket.
Attempting to free it with av_free() makes no sense.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/oggdec.c

@@ -817,7 +817,6 @@ retry:
     return psize;
 fail:
     av_free_packet(pkt);
-    av_free(pkt);
     return AVERROR(ENOMEM);
 }