FS-8344: mod_opus: toggle FEC on the last frame which is to be packed, so that

FEC will be only present on the first SILK frame of the next Opus frame (Opus repacketization).

src/mod/codecs/mod_opus/mod_opus.c

@@ -874,7 +874,6 @@ static switch_status_t switch_opus_encode_repacketize(switch_codec_t *codec,
 		if (codec->implementation->microseconds_per_packet / 1000 == 100) { /* 100 ms = 20 ms * 5 . because there is no 50 ms frame in Opus */
 				nb_frames = 5;
 		}
-		toggle_fec = 1;
 	}
 	frame_size = (decoded_data_len / 2) / nb_frames;
 	if((frame_size * nb_frames) != context->enc_frame_size) {
@@ -884,12 +883,19 @@ static switch_status_t switch_opus_encode_repacketize(switch_codec_t *codec,
 	opus_repacketizer_init(rp);
 	dec_ptr_buf = (int16_t *)decoded_data;
 	for (i = 0; i < nb_frames; i++) {
+		 /* set inband FEC ON or OFF for the next Opus frame */
+		if (i == (nb_frames - 1) && want_fec) { 
+			/* When FEC is enabled for Opus frame N, LBRR is stored during regular encoding of */ 
+			/* this Opus frame N, and this LBRR data will be packed with the regular encoding */
+			/* data of Opus frame N+1. We enable FEC on our last Opus frame which is to be packed, just */
+			/* to actually have it stored in the first Opus frame, that is when switch_opus_encode_repacketize() */
+			/* is called again to pack the next big 80,100 or 120 ms frame. */ 
+			toggle_fec = 1; /* FEC ON for the last frame */
+		}
+
+		opus_encoder_ctl(context->encoder_object, OPUS_SET_INBAND_FEC(toggle_fec));
 		bytes = opus_encode(context->encoder_object, (opus_int16 *) dec_ptr_buf, frame_size, enc_ptr_buf, len);
-		 /* set inband FEC off for the next frame to be packed, the current frame may contain FEC */
-		if (toggle_fec == 1) {
-			toggle_fec = 0;
-			opus_encoder_ctl(context->encoder_object, OPUS_SET_INBAND_FEC(toggle_fec));
-		} 
+
 		if (bytes < 0) {
 			switch_log_printf(SWITCH_CHANNEL_LOG, SWITCH_LOG_ERROR, "Encoder Error: %s Decoded Datalen %u Codec NumberChans %u" \
 							  "Len %u DecodedDate %p EncodedData %p ContextEncoderObject %p enc_frame_size: %d\n",