nlpsolver: Use System.arraycopy

Change-Id: I2550bf0799bd5ebc7f6ff299752fa90b7a31674d
Reviewed-on: https://gerrit.libreoffice.org/10806
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>