Assert in X11SalSystem::GetDisplayScreenPosSizePixel

If we detect that the display is using Xinerama, then we shouldn't be
able to pick a screen more than the number of screens available!
Therefore, assert if we meet this condition.

Also remove a comment decoration.

Change-Id: Ic8d2e0acf3e65825b76472adbd4a355e5435b6df
Reviewed-on: https://gerrit.libreoffice.org/7694
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>