A_SERIAL: Don't increment serial line number before saving

A_SERIAL: Don't increment serial line number before saving

The line number, as saved in the stats_serial structure, was used to
determine if the structure was empty or not: A value of 0 used to mean
that the structure was actually empty. As a consequence line #0 was
saved with the value 1, line #1 was saved with the value 2, etc.
This is no longer useful as structures are now dynamically allocated and
we know exactly how many of them we have.
So save now the real line number.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>