fdo#80462: Don't always increment the start row position.

Sometimes someone might attempt to sort only a single row.

Change-Id: Ie29d4cf7ec0bd3a5c945997083368b6ef6074268
(cherry picked from commit 5902dcf0995cdd0a6c1dbd1f9c21b0b2b3f5609f)
Reviewed-on: https://gerrit.libreoffice.org/10259
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>