fftools/ffmpeg: Fallback to duration if sample rate is unavailable

Regression since: af1761f7
Fixes: Division by 0
Fixes: ffmpeg_crash_1

Found-by: Thuan Pham, Marcel Böhme, Andrew Santosa and Alexandru Razvan Caciulescu with AFLSmart
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 16d8b13b3b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

ffmpeg.c

@@ -2725,8 +2725,12 @@ static int process_input_packet(InputStream *ist, const AVPacket *pkt, int no_eo
         ist->dts = ist->next_dts;
         switch (ist->dec_ctx->codec_type) {
         case AVMEDIA_TYPE_AUDIO:
-            ist->next_dts += ((int64_t)AV_TIME_BASE * ist->dec_ctx->frame_size) /
-                             ist->dec_ctx->sample_rate;
+            if (ist->dec_ctx->sample_rate) {
+                ist->next_dts += ((int64_t)AV_TIME_BASE * ist->dec_ctx->frame_size) /
+                                  ist->dec_ctx->sample_rate;
+            } else {
+                ist->next_dts += av_rescale_q(pkt->duration, ist->st->time_base, AV_TIME_BASE_Q);
+            }
             break;
         case AVMEDIA_TYPE_VIDEO:
             if (ist->framerate.num) {