ident: handle NULL ai_canonname

We call getaddrinfo() to try to convert a short hostname
into a fully-qualified one (to use it as an email domain).
If there isn't a canonical name, getaddrinfo() will
generally return either a NULL addrinfo list, or one in
which ai->ai_canonname is a copy of the original name.

However, if the result of gethostname() looks like an IP
address, then getaddrinfo() behaves differently on some
systems. On OS X, it will return a "struct addrinfo" with a
NULL ai_canonname, and we segfault feeding it to strchr().

This is hard to test reliably because it involves not only a
system where we we have to fallback to gethostname() to come
up with an ident, but also where the hostname is a number
with no dots. But I was able to replicate the bug by faking
a hostname, like:

    diff --git a/ident.c b/ident.c
    index e20a772..b790d28 100644
    --- a/ident.c
    +++ b/ident.c
    @@ -128,6 +128,7 @@ static void add_domainname(struct strbuf *out, int *is_bogus)
                     *is_bogus = 1;
                     return;
             }
    +        xsnprintf(buf, sizeof(buf), "1");
             if (strchr(buf, '.'))
                     strbuf_addstr(out, buf);
             else if (canonical_name(buf, out) < 0) {

and running "git var GIT_AUTHOR_IDENT" on an OS X system.

Before this patch it segfaults, and after we correctly
complain of the bogus "user@1.(none)" address (though this
bogus address would be suitable for non-object uses like
writing reflogs).

Reported-by: Jonas Thiel <jonas.lierschied@gmx.de>
Diagnosed-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ident.c

@@ -103,7 +103,7 @@ static int canonical_name(const char *host, struct strbuf *out)
 	memset (&hints, '\0', sizeof (hints));
 	hints.ai_flags = AI_CANONNAME;
 	if (!getaddrinfo(host, NULL, &hints, &ai)) {
-		if (ai && strchr(ai->ai_canonname, '.')) {
+		if (ai && ai->ai_canonname && strchr(ai->ai_canonname, '.')) {
 			strbuf_addstr(out, ai->ai_canonname);
 			status = 0;
 		}