use first audio/subtitle track when best_track could not be confirmed

[nairyosangha]

When dealing with files which contain multiple unnamed subtitle (or audio) tracks, the best_track function will not return anything and the ffmpeg will use -s (or -a) as map parameter. According to the ffmpeg docs this will select all suitable streams. This causes the subtitle extract to fail with the error below

[srt @ 0x558672431600] SRT supports only a single subtitles stream. Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument Error initializing output stream 0:1 --

[tatsumoto-ren]

Thank you!