mmdemux: dont set pkt->size to an invalid value.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 0c97fd336e17535239ab44d755a0d957dc2688f3)
diff --git a/libavformat/mm.c b/libavformat/mm.c
index 6c621ab..1868e90 100644
--- a/libavformat/mm.c
+++ b/libavformat/mm.c
@@ -162,7 +162,6 @@
         case MM_TYPE_AUDIO :
             if (av_get_packet(s->pb, pkt, length)<0)
                 return AVERROR(ENOMEM);
-            pkt->size = length;
             pkt->stream_index = 1;
             pkt->pts = mm->audio_pts++;
             return 0;