Avoid "negative" average buffering

Average buffering may not be negative.

Since dropping samples instantly reduces buffering, it is applied
immediately to the average value.

However, if an audio output issue causes the SDL callback to be called
too infrequently, samples will (silently) not be consumed as expected.
As this will cause samples to be dropped due to overbuffering, the
average value may drop below 0, which makes no sense.

app/src/audio_player.c

@@ -266,6 +266,14 @@ sc_audio_player_frame_sink_push(struct sc_frame_sink *sink,
 
     // The compensation must apply instantly, it must not be smoothed
     ap->avg_buffering.avg += instant_compensation + inserted_silence - dropped;
+    if (ap->avg_buffering.avg < 0) {
+        // Negative buffering makes no sense
+        // This may happen when the consumer does not consume at the expected
+        // rate (the SDL calback is not called often enough, which is an audio
+        // output issue), causing many samples to be dropped due to
+        // overbuffering.
+        ap->avg_buffering.avg = 0;
+    }
 
     // However, the buffering level must be smoothed
     sc_average_push(&ap->avg_buffering, can_read);