plugins/radius: fix segfault during shutdown. (ppp-project#455)

ppp_get_ifname() is the wrong thing to use in this slprintf call as
it returns an int which is the length of the interface name, not
a pointer to the interface name, which is what ppp_ifname()
returns.

Closes: ppp-project#454

Signed-off-by: Jaco Kroon <jaco@uls.co.za>

pppd/plugins/radius/radattr.c

@@ -114,7 +114,7 @@ cleanup(void *opaque, int arg)
 {
     char fname[512];
 
-    slprintf(fname, sizeof(fname), "/var/run/radattr.%s", ppp_get_ifname(NULL,0));
+    slprintf(fname, sizeof(fname), "/var/run/radattr.%s", ppp_ifname());
     (void) remove(fname);
     dbglog("RADATTR plugin removed file %s.", fname);
 }