avidec/guess_ni_flag: fix harmless integer overflow

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

libavformat/avidec.c

@@ -1341,7 +1341,7 @@ static int guess_ni_flag(AVFormatContext *s){
     if (last_start > first_end)
         return 1;
     idx= av_mallocz(sizeof(*idx) * s->nb_streams);
-    for (min_pos=pos=0; min_pos!=INT64_MAX; pos= min_pos+1) {
+    for (min_pos=pos=0; min_pos!=INT64_MAX; pos= min_pos+1LU) {
         int64_t max_dts = INT64_MIN/2, min_dts= INT64_MAX/2;
         min_pos = INT64_MAX;