This fixes a buffer overflow writing over the end of pCaretXArray,
which can happen e.g. when drawing mnemonics in the password dialog.
Based on a similar calculation of nCurrIdx found in
GenericSalLayout::GetCaretPositions().

Change-Id: I7d723cf8cfaeb66f340c7d9ea5b3bc728c6d6209
Reviewed-on: https://gerrit.libreoffice.org/19385Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: jan iversen <jani@documentfoundation.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>