BUG/MINOR: cfgparse: fix NULL ptr dereference in cfg_parse_peers

BUG/MINOR: cfgparse: fix NULL ptr dereference in cfg_parse_peers

When "peers" keyword is followed by more than one argument and it's the first
"peers" section in the config, cfg_parse_peers() detects it and exits with
"ERR_ALERT|ERR_FATAL" err_code.

So, upper layer parser, parse_cfg(), continues and parses the next keyword
"peer" and then he tries to check the global cfg_peers, which should contain
"my_cluster". The global cfg_peers is still NULL, because after alerting a user
in alertif_too_many_args, cfg_parse_peers() exited.

	peers my_cluster __some_wrong_data__
	peer haproxy1 1.1.1.1 1000

In order to fix this, let's add ERR_ABORT, if "peers" keyword is followed by
more than one argument. Like this parse_cfg() will stops immediately and
terminates haproxy with "too many args for peers my_cluster..." alert message.

It's more reliable, than add checks "if (cfg_peers !=NULL)" in "peer"
subparser, as we may have many "peers" sections.

	peers my_another_cluster
	peer haproxy1 1.1.1.2 1000

	peers my_cluster  __some_wrong_data__
	peer haproxy1 1.1.1.1 1000

In addition, for the example above, parse_cfg() will parse all configuration
until the end and only then terminates haproxy with the alert
"too many args...". Peer haproxy1 will be wrongly associated with
my_another_cluster.

This fixes the issue #2872.
This should be backported in all stable versions.

(cherry picked from commit 390df282c1ac4605273abfeb82c97fad205b7294)
Signed-off-by: Willy Tarreau <w@1wt.eu>