Mailmerge shows wrong number of emails to be sent

The progress bar is ok, but the text is displayed with the total
number of registers instead of the amount selected to be used.

Change-Id: I81da0e7ba9f193366e12cbf81118e36a43bcaad4
Signed-off-by: Rodolfo Ribeiro Gomes <rodolforg@gmail.com>
Reviewed-on: https://gerrit.libreoffice.org/6810
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit eda8111fdf9b1c69935237106a73d1c53c2776e1)