operserv/akill: fix crash bug introduced in a2126db3

/os AKILL ADD would segfault as it'd try to compare the non-existent
first argument to "FORCE".
get_kline_userhost: use caller-supplied buffers

This allows hooks to rewrite the user@host to be klined more
arbitrarily, e.g. turning uid12345@whatever.irccloud.com into
?id12345@*.irccloud.com or similar.