BUG/MINOR: cfgparse-listen: do not emit extraneous line in rule order warnings

BUG/MINOR: cfgparse-listen: do not emit extraneous line in rule order warnings

Some functions such as tcp_parse_tcp_req() are able to emit their own
warnings by relying on warnif_misplaced_*() which directly prints the
warning. However when doing so they still increment the warning counter
which makes cfg_parse_listen() try to emit it, except that what's in the
variable is NULL, so we end up with:

  [WARNING]  (260) : config : parsing [/etc/haproxy/haproxy.cfg:17] : (null)

Let's just check the errmsg variable before printing the error. If it's
NULL, it's because the message was already printed.

This can be backported to all branches.