lavf: fix signed overflow in avformat_find_stream_info()

On the first iteration through this code, last_dts is always
INT64_MIN (AV_NOPTS_VALUE) and the subtraction overflows in
an invalid manner.  Although the result is only used if the
input values are valid, performing the subtraction is still
not allowed in a strict environment.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/utils.c

@@ -2358,9 +2358,9 @@ int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
         }
         {
             int64_t last = st->info->last_dts;
-            int64_t duration= pkt->dts - last;
 
-            if(pkt->dts != AV_NOPTS_VALUE && last != AV_NOPTS_VALUE && duration>0){
+            if(pkt->dts != AV_NOPTS_VALUE && last != AV_NOPTS_VALUE && pkt->dts > last){
+                int64_t duration= pkt->dts - last;
                 double dur= duration * av_q2d(st->time_base);
 
 //                if(st->codec->codec_type == AVMEDIA_TYPE_VIDEO)