[core] CPlayListPlayer::Reset dereferences the GUI unguarded, while SetRepeat two functions away null-checks it

[malard]

CPlayListPlayer::Reset() dereferences the GUI without checking it, two functions away from one
that does check. A test cannot call it, so the production route into the defect fixed by
xbmc#28873 cannot be driven from a test.

Verified against origin/master at 858c6ee4b6.

The inconsistency

Unguarded - xbmc/PlayListPlayer.cpp:517-526:

void CPlayListPlayer::Reset()
{
  m_iCurrentSong = -1;
  ...
  CGUIMessage msg(GUI_MSG_PLAYLIST_CHANGED, 0, 0);
  CServiceBroker::GetGUI()->GetWindowManager().SendMessage(msg);   // :525
}

Guarded - SetRepeat, PlayListPlayer.cpp:668:

if (CServiceBroker::GetGUI() != nullptr)
{
  CGUIMessage msg(GUI_MSG_PLAYLIST_CHANGED, 0, 0);
  CServiceBroker::GetGUI()->GetWindowManager().SendMessage(msg);
}

Across the file: 19 unguarded GetGUI()-> dereferences against 2 guarded ones. Whichever way
this should go, it is currently both.

Why it matters beyond tidiness

ClearPlaylist calls Reset(), which is how the current index reaches -1 in the application.
The test added by xbmc#28873 has to set that index directly instead, so it covers
GetNextItemIdx's handling of -1 - the defect - while the link back to ClearPlaylist rests on
reading the code. Guarding Reset() would let a test drive the real path.

Decision needed

Guarding Reset() alone is three lines and unblocks the above. Whether the other 18 sites follow,
or the class instead declares that it requires a GUI, is the larger question and is why this is
filed rather than fixed.