Fix potential NULL pointer deref in rblchk(), observed via segfault lately

I'm not sure why this one only started popping up now, but h_addr_list is a
NULL-terminated list and it makes perfect sense that the first entry could
be the NULL-terminator.

gethostbyname is obsolete/deprecated and we should address that in a separate
commit.

src/sbbs3/mailsrvr.c

@@ -1833,6 +1833,9 @@ static ulong rblchk(SOCKET sock, const char* prot, union xp_sockaddr *addr, cons
 	if((host=gethostbyname(name))==NULL)
 		return(0);
 
+	if(host->h_addr_list[0] == NULL)
+		return 0;
+
 	dnsbl_result.s_addr = *((ulong*)host->h_addr_list[0]);
 	lprintf(LOG_INFO,"%04d %s DNSBL Query: %s resolved to: %s"
 		,sock,prot,name,inet_ntoa(dnsbl_result));