runner: Dereference of the result of readLine() without nullcheck

Change-Id: I3d02436446a79137ce42b1a190c9e3985fbb9246
Reviewed-on: https://gerrit.libreoffice.org/12135
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>