Convert unused seekpoints in template to placeholders (#754)

See https://lists.xiph.org/pipermail/flac-dev/2024-October/006671.html
for details

src/libFLAC/stream_encoder.c

@@ -3178,14 +3178,15 @@ void update_metadata_(const FLAC__StreamEncoder *encoder)
 				FLAC__STREAM_METADATA_STREAMINFO_BITS_PER_SAMPLE_LEN
 				- 4
 			) / 8;
+		FLAC__uint64 samples_uint36 = samples;
 		if(samples > (FLAC__U64L(1) << FLAC__STREAM_METADATA_STREAMINFO_TOTAL_SAMPLES_LEN))
-			samples = 0;
+			samples_uint36 = 0;
 
-		b[0] = ((FLAC__byte)(bps-1) << 4) | (FLAC__byte)((samples >> 32) & 0x0F);
-		b[1] = (FLAC__byte)((samples >> 24) & 0xFF);
-		b[2] = (FLAC__byte)((samples >> 16) & 0xFF);
-		b[3] = (FLAC__byte)((samples >> 8) & 0xFF);
-		b[4] = (FLAC__byte)(samples & 0xFF);
+		b[0] = ((FLAC__byte)(bps-1) << 4) | (FLAC__byte)((samples_uint36 >> 32) & 0x0F);
+		b[1] = (FLAC__byte)((samples_uint36 >> 24) & 0xFF);
+		b[2] = (FLAC__byte)((samples_uint36 >> 16) & 0xFF);
+		b[3] = (FLAC__byte)((samples_uint36 >> 8) & 0xFF);
+		b[4] = (FLAC__byte)(samples_uint36 & 0xFF);
 		if((seek_status = encoder->private_->seek_callback(encoder, encoder->protected_->streaminfo_offset + total_samples_byte_offset, encoder->private_->client_data)) != FLAC__STREAM_ENCODER_SEEK_STATUS_OK) {
 			if(seek_status == FLAC__STREAM_ENCODER_SEEK_STATUS_ERROR)
 				encoder->protected_->state = FLAC__STREAM_ENCODER_CLIENT_ERROR;
@@ -3231,6 +3232,11 @@ void update_metadata_(const FLAC__StreamEncoder *encoder)
 	if(0 != encoder->private_->seek_table && encoder->private_->seek_table->num_points > 0 && encoder->protected_->seektable_offset > 0) {
 		uint32_t i;
 
+		/* Convert unused seekpoints to placeholders */
+		for(i = 0; i < encoder->private_->seek_table->num_points; i++)
+			if(encoder->private_->seek_table->points[i].sample_number > samples)
+				encoder->private_->seek_table->points[i].sample_number = FLAC__STREAM_METADATA_SEEKPOINT_PLACEHOLDER;
+
 		FLAC__format_seektable_sort(encoder->private_->seek_table);
 
 		FLAC__ASSERT(FLAC__format_seektable_is_legal(encoder->private_->seek_table));