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>