BUG/MINOR: http_ext/7239: ipv6 dumping relies on out of scope variables

BUG/MINOR: http_ext/7239: ipv6 dumping relies on out of scope variables

In http_build_7239_header_nodename(), ip6 address dumping is performed
at a single place to prevent code duplication:
A goto statement combined with a local pointer variable (ip6_addr) were used
to perform ipv6 dump from different calling places inside the function.

However, when the goto was performed (ie: sample expression handling),
ip6_addr pointer was assigned to limited scope variable's address that is not
valid within the dumping code.
Because of this, we have an undefined behavior that could result in a bug or
a crash depending on the platform that is running haproxy.

This was found by Coverity (GH #2018)

To fix this, we add a simple ip6 printing helper that takes the ip6_addr
pointer as an argument.
This prevents any scope related bug as the function is executed under the
proper context.

if/else guards inside the function were reviewed to make sure that the goto
removal won't affect existing behavior.

----------

No backport needed, except if the commit ("MINOR: proxy/http_ext: introduce
proxy forwarded option") is backported.

Given that this commit needs to be backported with
"MINOR: proxy/http_ext: introduce proxy forwarded option", We're using it as a
reminder for another bug that was introduced with
"MINOR: proxy/http_ext: introduce proxy forwarded option" but has been silently
fixed since with
"MEDIUM: proxy/http_ext: implement dynamic http_ext".

If "MINOR: proxy/http_ext: introduce proxy forwarded option" needs to be
backported without
"MEDIUM: proxy/http_ext: implement dynamic http_ext", you should manually apply
the following patch on top of it:

    |  diff --git a/src/http_ext.c b/src/http_ext.c
    |  index fcb5a07bc..3921357a3 100644
    |  --- a/src/http_ext.c
    |  +++ b/src/http_ext.c
    |  @@ -609,7 +609,7 @@ static inline void http_build_7239_header_node(struct buffer *out,
    |   	if (forby->np_mode)
    |   		chunk_appendf(out, "\"");
    |   	offset_save = out->data;
    |  -	http_build_7239_header_node(out, s, curproxy, addr, &curproxy->http.fwd.p_by);
    |  +	http_build_7239_header_nodename(out, s, curproxy, addr, forby);
    |   	if (offset_save == out->data) {
    |   		/* could not build nodename, either because some
    |   		 * data is not available or user is providing bad input
    |  @@ -619,7 +619,7 @@ static inline void http_build_7239_header_node(struct buffer *out,
    |   	if (forby->np_mode) {
    |   		chunk_appendf(out, ":");
    |   		offset_save = out->data;
    |  -		http_build_7239_header_nodeport(out, s, curproxy, addr, &curproxy->http.fwd.p_by);
    |  +		http_build_7239_header_nodeport(out, s, curproxy, addr, forby);
    |   		if (offset_save == out->data) {
    |   			/* could not build nodeport, either because some data is
    |   			 * not available or user is providing bad input

(If you don't, forwarded option won't work properly and will crash
haproxy (stack overflow) when building 'for' or 'by' parameter)