[ffmpeg] Add `aac_adtstoasc` when merging if needed

Related: #1039

yt_dlp/YoutubeDL.py

@@ -2798,6 +2798,7 @@ class YoutubeDL(object):
                                     'f%s' % f['format_id'], new_info['ext'])
                                 if not self._ensure_dir_exists(fname):
                                     return
+                                f['filepath'] = fname
                                 downloaded.append(fname)
                             partial_success, real_download = self.dl(fname, new_info)
                             info_dict['__real_download'] = info_dict['__real_download'] or real_download

yt_dlp/postprocessor/ffmpeg.py

@@ -732,7 +732,9 @@ class FFmpegMergerPP(FFmpegPostProcessor):
         args = ['-c', 'copy']
         for (i, fmt) in enumerate(info['requested_formats']):
             if fmt.get('acodec') != 'none':
-                args.extend(['-map', '%u:a:0' % (i)])
+                args.extend(['-map', f'{i}:a:0'])
+                if self.get_audio_codec(fmt['filepath']) == 'aac':
+                    args.extend([f'-bsf:{i}:a:0', 'aac_adtstoasc'])
             if fmt.get('vcodec') != 'none':
                 args.extend(['-map', '%u:v:0' % (i)])
         self.to_screen('Merging formats into "%s"' % filename)