so it can be a vector instead, and by using vector::at() instead of
matrix::at() vector bounds checking is performed, unlike matrix::at()
which does no checking

Change-Id: Ic767c2dd884bffbf1cdff65c0980b21170612f4d
Reviewed-on: https://gerrit.libreoffice.org/58396
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>