#910 External audio file blocks audio track menu selection

[ValdikSS]

I'm using smplayer 18.10.0 (revision 9144), MPV 0.29.1, from Fedora rpmfusion repositories.

Loading external audio track for the video file blocks audio track selection menu. After loading external audio file, selection menu is grayed out with only a single external audio file option.
Switching audio tracks with mpv hotkey works fine.

Steps to reproduce:
1. Open any video file with one or more audio tracks
2. Audio → Load external file → choose any audio file
3. Try to change track in Audio → Tracks menu

Expected result:
After loading external file, there's additional audio track in "Audio → Tracks" menu. You can switch audio tracks.

Actual result:
External audio file replaces all audio tracks inside video file. You can't switch audio tracks. "Next audio track" hotkey still works properly, it switches between internal and external audio tracks.