[DVDPlayer/Subs] Save the subtitle number at the end of the OpenSubtitleStream() method.
Regardless of whether the subtitle has been opened successfully or not.
[GuiDialog/PlayerControls] Use CMediaSettings::Get().GetCurrentVideoSettings().m_SubtitleStream instead of g_application.m_pPlayer->GetSubtitle().
The latter one returns the maximal index if the current subtitle could not be opened. This yields wrong info about the subtitle.
Merge pull request #2825 from ace20022/sub_fixes

[Fix/Subtitles] Fix wrong indices of erroneous subtitles.