BUG/MINOR: cfgparse/peers: fix inconsistent check for missing peer server

BUG/MINOR: cfgparse/peers: fix inconsistent check for missing peer server

In the "peers" section parser, right after parse_server() is called, we
used to check whether the curpeers->peers_fe->srv pointer was set or not
to know if parse_server() successfuly added a server to the peers proxy,
server that we can then associate to the new peer.

However the check is wrong, as curpeers->peers_fe->srv points to the
last added server, if a server was successfully added before the
failing one, we cannot detect that the last parse_server() didn't
add a server. This is known to cause bug with bad "peer"/"server"
statements.

To fix the issue, we save a pointer on the last known
curpeers->peers_fe->srv before parse_server() is called, and we then
compare the save with the pointer after parse_server(), if the value
didn't change, then parse_server() didn't add a server. This makes
the check consistent in all situations.

It should be backported to all stable versions.

(cherry picked from commit 2560ab892f355e958007b287946f787b578d3131)
Signed-off-by: Willy Tarreau <w@1wt.eu>