Do not use C++11 std::vector.data(), use an alternative instead

Note that there is no need to check for empty(), because
matrix is always initialized with all empty elements in this code.

Change-Id: If05b3b12d7209b99fcbb09f7c97e74476fb9ce06
Reviewed-on: https://gerrit.libreoffice.org/7098
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Tested-by: Kohei Yoshida <libreoffice@kohei.us>