demux_lavf: simplify replaygain export

* Early exit if there is no useful data in the AVReplayGain
  structure (FFmpeg does check that gain of at least one thing is
  not INT32_MIN, but leaves the peak unchecked so it can be zero).
* Less depth in the if structure.

demux/demux_lavf.c

@@ -618,31 +618,33 @@ static void export_replaygain(demuxer_t *demuxer, struct sh_stream *sh,
     if (!av_rgain)
         return;
 
-    struct replaygain_data *rgain = talloc_ptrtype(demuxer, rgain);
+    bool track_data_available =
+        av_rgain->track_gain != INT32_MIN && av_rgain->track_peak != 0;
+    bool album_data_available =
+        av_rgain->album_gain != INT32_MIN && av_rgain->album_peak != 0;
+
+    if (!track_data_available && !album_data_available)
+        return;
 
+    struct replaygain_data *rgain = talloc_ptrtype(demuxer, rgain);
     rgain->track_gain = rgain->album_gain = 0;
     rgain->track_peak = rgain->album_peak = 1;
 
     // Set values in *rgain, using track gain as a fallback for album gain
     // if the latter is not present. This behavior matches that in
     // demux/demux.c's decode_rgain; if you change this, please make
     // equivalent changes there too.
-    if (av_rgain->track_gain != INT32_MIN && av_rgain->track_peak != 0.0) {
-        // Track gain is defined.
-        rgain->track_gain = av_rgain->track_gain / 100000.0f;
-        rgain->track_peak = av_rgain->track_peak / 100000.0f;
+    if (track_data_available) {
+        rgain->track_gain = rgain->album_gain =
+            av_rgain->track_gain / 100000.0f;
+        rgain->track_peak = rgain->album_peak =
+            av_rgain->track_peak / 100000.0f;
+    }
 
-        if (av_rgain->album_gain != INT32_MIN &&
-            av_rgain->album_peak != 0.0)
-        {
-            // Album gain is also defined.
-            rgain->album_gain = av_rgain->album_gain / 100000.0f;
-            rgain->album_peak = av_rgain->album_peak / 100000.0f;
-        } else {
-            // Album gain is undefined; fall back to track gain.
-            rgain->album_gain = rgain->track_gain;
-            rgain->album_peak = rgain->track_peak;
-        }
+    // If album data is actually available, fill it in with proper values.
+    if (album_data_available) {
+        rgain->album_gain = av_rgain->album_gain / 100000.0f;
+        rgain->album_peak = av_rgain->album_peak / 100000.0f;
     }
 
     // This must be run only before the stream was added, otherwise there