fftools/ffmpeg: stop using AVCodecContext.sample_rate in decode_audio()

Use the decoded frame's sample_rate instead, which is the authoritative
value.

Drop a now-obsolete check validating AVCodecContext.sample_rate.

fftools/ffmpeg.c

@@ -2017,11 +2017,6 @@ static int decode_audio(InputStream *ist, AVPacket *pkt, int *got_output,
     if (ret < 0)
         *decode_failed = 1;
 
-    if (ret >= 0 && avctx->sample_rate <= 0) {
-        av_log(avctx, AV_LOG_ERROR, "Sample rate %d invalid\n", avctx->sample_rate);
-        ret = AVERROR_INVALIDDATA;
-    }
-
     if (ret != AVERROR_EOF)
         check_decode_result(ist, got_output, ret);
 
@@ -2034,9 +2029,9 @@ static int decode_audio(InputStream *ist, AVPacket *pkt, int *got_output,
     /* increment next_dts to use for the case where the input stream does not
        have timestamps or there are multiple frames in the packet */
     ist->next_pts += ((int64_t)AV_TIME_BASE * decoded_frame->nb_samples) /
-                     avctx->sample_rate;
+                     decoded_frame->sample_rate;
     ist->next_dts += ((int64_t)AV_TIME_BASE * decoded_frame->nb_samples) /
-                     avctx->sample_rate;
+                     decoded_frame->sample_rate;
 
     if (decoded_frame->pts != AV_NOPTS_VALUE) {
         decoded_frame_tb   = ist->st->time_base;
@@ -2054,8 +2049,10 @@ static int decode_audio(InputStream *ist, AVPacket *pkt, int *got_output,
         ist->prev_pkt_pts = pkt->pts;
     if (decoded_frame->pts != AV_NOPTS_VALUE)
         decoded_frame->pts = av_rescale_delta(decoded_frame_tb, decoded_frame->pts,
-                                              (AVRational){1, avctx->sample_rate}, decoded_frame->nb_samples, &ist->filter_in_rescale_delta_last,
-                                              (AVRational){1, avctx->sample_rate});
+                                              (AVRational){1, decoded_frame->sample_rate},
+                                              decoded_frame->nb_samples,
+                                              &ist->filter_in_rescale_delta_last,
+                                              (AVRational){1, decoded_frame->sample_rate});
     ist->nb_samples = decoded_frame->nb_samples;
     err = send_frame_to_filters(ist, decoded_frame);