[Xymon] xymonnet - fatal signal caught

[Mark Felder]

Another FreeBSD user hit this and helped track this down. This crash is
specific to 4.3.18 but affects all platforms.

http://sourceforge.net/p/xymon/code/7484/tree//branches/4.3.18/xymonnet/dns2.c?diff=516c17fd34309d2eb14bcb64:7483

addtobuffer_many() in case T_AAAA is a variadic function that needs to
terminate in a NULL.

This patch fixes the crash caused by any DNS checks which return AAAA
records.

https://svnweb.freebsd.org/ports/head/net-mgmt/xymon-server/files/patch-xymonnet_dns2.c?view=markup&pathrev=376300