When looking for the last layout row used, we need to look at the layout

When looking for the last layout row used, we need to look at the layout
row used for the previous data line containing data, not at the previous
data line outright, which might be a horizontal ruler.  If it is, do not
restart from the first layout row but still proceed to the next data row,
which may have been just read from T&.

Bug originally reported by Oliver dot Corff at email dot de
on groff at gnu dot org:
https://lists.gnu.org/archive/html/groff/2021-03/msg00003.html
and forwarded to me by bentley@.

Patch OK'ed by bentley@ back in April.