host_callback: Fall back to AF_INET on searching with AF_UNSPEC

Previously, when an ares_gethostbyname() searched with AF_UNSPEC and the
first AF_INET6 call only returned CNAMEs, the host_callback never
retried AF_INET.

This patch makes sure than on ARES_SUCCESS, the result of AF_INET6 is
taken as authoritative only if the result contains some addresses.

ares_gethostbyname.c

@@ -188,8 +188,9 @@ static void host_callback(void *arg, int status, int timeouts,
       else if (hquery->sent_family == AF_INET6)
         {
           status = ares_parse_aaaa_reply(abuf, alen, &host, NULL, NULL);
-          if ((status == ARES_ENODATA || status == ARES_EBADRESP) &&
-               hquery->want_family == AF_UNSPEC) {
+          if ((status == ARES_ENODATA || status == ARES_EBADRESP ||
+               (status == ARES_SUCCESS && host && host->h_addr_list[0] == NULL)) &&
+                hquery->want_family == AF_UNSPEC) {
             /* The query returned something but either there were no AAAA
                records (e.g. just CNAME) or the response was malformed.  Try
                looking up A instead. */