rtadvd: don't clobber errno

[Florian Obser]

... we want to log_warn what went wrong.
OK?

diff --git if.c if.c
index 4683bcffac9..c4a4fea1984 100644
--- if.c
+++ if.c
@@ -111,7 +111,7 @@ if_nametosdl(char *name)
 int
 if_getmtu(char *name)
 {
-       int             s;
+       int             s, save_errno;
        struct ifreq    ifr;
        u_long          mtu = 0;
 
@@ -123,7 +123,9 @@ if_getmtu(char *name)
                        fatalx("strlcpy");
                if (ioctl(s, SIOCGIFMTU, (char *)&ifr) >= 0)
                        mtu = ifr.ifr_mtu;
+               save_errno = errno;
                close(s);
+               errno = save_errno;
        }
 
        return (mtu);

-- 
I'm not entirely sure you are real.