minor cleanup in ScHeaderControl: nCount is always greater than 0

Change-Id: Ia50265e397704131dc948bc2bcbf80a442054b71
Signed-off-by: Rodolfo Ribeiro Gomes <rodolforg@gmail.com>
Reviewed-on: https://gerrit.libreoffice.org/6538
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>