Fix asset crash interleaving with active encoders

Apparently the audio isn't guaranteed to start up past the first video
frame, so it would trigger that assert (which I'm glad I put in).  I
didn't originally have this happen when I was testing because my audio
buffering was not at the default value and didn't trigger it to occur.
A blunder on my part, and once again a fine example of how you should
never make assumptions about possible code path.

libobs/obs-output.c

@@ -686,12 +686,10 @@ static struct encoder_packet *find_first_packet_type(struct obs_output *output,
 			return packet;
 	}
 
-	/* should never get here for this particular function */
-	assert(0);
 	return NULL;
 }
 
-static void initialize_interleaved_packets(struct obs_output *output)
+static bool initialize_interleaved_packets(struct obs_output *output)
 {
 	struct encoder_packet *video;
 	struct encoder_packet *audio;
@@ -699,6 +697,14 @@ static void initialize_interleaved_packets(struct obs_output *output)
 	video = find_first_packet_type(output, OBS_ENCODER_VIDEO);
 	audio = find_first_packet_type(output, OBS_ENCODER_AUDIO);
 
+	if (!audio)
+		output->received_audio = false;
+	if (!video)
+		output->received_video = false;
+	if (!audio || !video) {
+		return false;
+	}
+
 	/* get new offsets */
 	output->video_offset = video->dts;
 	output->audio_offset = audio->dts;
@@ -713,6 +719,8 @@ static void initialize_interleaved_packets(struct obs_output *output)
 			&output->interleaved_packets.array[i];
 		apply_interleaved_packet_offset(output, packet);
 	}
+
+	return true;
 }
 
 static inline void insert_interleaved_packet(struct obs_output *output,
@@ -764,11 +772,13 @@ static void interleave_packets(void *data, struct encoder_packet *packet)
 	if (output->received_audio && output->received_video) {
 		if (!was_started) {
 			prune_interleaved_packets(output);
-			initialize_interleaved_packets(output);
-			resort_interleaved_packets(output);
+			if (initialize_interleaved_packets(output)) {
+				resort_interleaved_packets(output);
+				send_interleaved(output);
+			}
+		} else {
+			send_interleaved(output);
 		}
-
-		send_interleaved(output);
 	}
 
 	pthread_mutex_unlock(&output->interleaved_mutex);