avformat/utils: Check timebase before use in estimate_timings()

Fixes: division by 0
Fixes: 15480/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5746727434321920

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/utils.c

@@ -2953,6 +2953,7 @@ static void estimate_timings(AVFormatContext *ic, int64_t old_offset)
         AVStream av_unused *st;
         for (i = 0; i < ic->nb_streams; i++) {
             st = ic->streams[i];
+            if (st->time_base.den)
             av_log(ic, AV_LOG_TRACE, "stream %d: start_time: %0.3f duration: %0.3f\n", i,
                    (double) st->start_time * av_q2d(st->time_base),
                    (double) st->duration   * av_q2d(st->time_base));