ffmpeg: Fix integer overflow with cur_dts being AV_NOPTS_VALUE

Since de0e219a cur_dts is sometimes invalid

ffmpeg.c

@@ -3382,8 +3382,12 @@ static OutputStream *choose_output(void)
 
     for (i = 0; i < nb_output_streams; i++) {
         OutputStream *ost = output_streams[i];
-        int64_t opts = av_rescale_q(ost->st->cur_dts, ost->st->time_base,
+        int64_t opts = ost->st->cur_dts == AV_NOPTS_VALUE ? INT64_MIN :
+                       av_rescale_q(ost->st->cur_dts, ost->st->time_base,
                                     AV_TIME_BASE_Q);
+        if (ost->st->cur_dts == AV_NOPTS_VALUE)
+            av_log(NULL, AV_LOG_DEBUG, "cur_dts is invalid (this is harmless if it occurs once at the start per stream)\n");
+
         if (!ost->finished && opts < opts_min) {
             opts_min = opts;
             ost_min  = ost->unavailable ? NULL : ost;