[823] forwarder setting with NetBSD/sparc

[Jun-ichiro itojun Hagino]

	with BIND823 on on NetBSD/sparc, there are problem reports like this:
	http://www.NetBSD.org/cgi-bin/query-pr-single.pl?number=12128
	to sumamrize, if we set multiple forwarders into named.conf, named
	will coredump. (there's some report that it does not happen on i386).

	i looked at the code briefly, and found a possible source of problem.
	it would be good if you can check if my reasoning is correct.


	global variable fwddata[] keeps the all list forwarder data,
	created in find_forwarder().  fwddata->ref_count (in struct fwddata)
	does not count the pointer from fwddata.

	when ftp->fwddata->ref_count reaches 0, we free ftp->fwddata
	(ns_config.c near line 2400).  we do not cleanup pointers from
	fwddata[] for this particular item, therefore, we leave a dangling
	pointer in fwddata[].

	next time we visit find_forwarder(), we have possibility to touch
	dangling pointer in fwddata[].  the pointer would be returned to
	the caller of find_forwarder if we are (un)lucky, and the pointer
	will be kept into ftp->fwddata.  when the region gets reused, we will
	see coredump.


	solution would be to cleanup pointer from fwddata[], when a fwddata
	item goes away.

itojun