[BUG] checks: don't log backend down for all zero-weight servers

In a down backend, when a zero-weight server is lost, a new
"backend down" message was emitted and the down transition of that
backend was wrongly increased. This change ensures that we don't
count that transition again.

This patch should be backported to 1.3.
(cherry picked from commit 60efc5f745b5fa70d811f977727592e47e32a281)
1 file changed