avformat/movenc: Check packet in mov_write_single_packet() too

Fixes assertion failure

Found-by: durandal117
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 28343139)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/movenc.c

@@ -4901,6 +4901,10 @@ static int mov_write_single_packet(AVFormatContext *s, AVPacket *pkt)
         int64_t frag_duration = 0;
         int size = pkt->size;
 
+        int ret = check_pkt(s, pkt);
+        if (ret < 0)
+            return ret;
+
         if (mov->flags & FF_MOV_FLAG_FRAG_DISCONT) {
             int i;
             for (i = 0; i < s->nb_streams; i++)