fix for rt_newmaddmsg panic()

[Andrew Atrens]

On unloading the iwi driver I get a panic in rt_newaddrmsg.

Looks like version 1.20 of rtsock.c (HEAD) is missing a 
test found in version 1.19 of rtsock.c (STABLE).

Reapplying the missing (removed) test fixes the panic.

Andrew.
--- rtsock.c	2005-02-23 16:39:22.000000000 -0500
+++ rtsock.c.old	2005-02-23 16:39:12.000000000 -0500
@@ -972,7 +972,7 @@
 
 	bzero(&rtinfo, sizeof(struct rt_addrinfo));
 	rtinfo.rti_ifaaddr = ifma->ifma_addr;
-	if (ifp != NULL && TAILQ_FIRST(&ifp->if_addrhead) != NULL)
+	if (ifp != NULL)
 		rtinfo.rti_ifpaddr = TAILQ_FIRST(&ifp->if_addrhead)->ifa_addr;
 	else
 		rtinfo.rti_ifpaddr = NULL;