avformat/mxfdec: Fix Sign error in mxf_read_primer_pack()

Fixes: 20170829B.mxf

Co-Author: 张洪亮(望初)" <wangchu.zhl@alibaba-inc.com>
Found-by: Xiaohei and Wangchu from Alibaba Security Team
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mxfdec.c

@@ -500,7 +500,7 @@ static int mxf_read_primer_pack(void *arg, AVIOContext *pb, int tag, int size, U
         avpriv_request_sample(pb, "Primer pack item length %d", item_len);
         return AVERROR_PATCHWELCOME;
     }
-    if (item_num > 65536) {
+    if (item_num > 65536 || item_num < 0) {
         av_log(mxf->fc, AV_LOG_ERROR, "item_num %d is too large\n", item_num);
         return AVERROR_INVALIDDATA;
     }