gprs_ns2_fr: free_bind(): first do the NULL check before using members

Change-Id: I80f72ada15b477d735bf5aee36a5d67e80eb0136
diff --git a/src/gb/gprs_ns2_fr.c b/src/gb/gprs_ns2_fr.c
index f1a5576..78abddc 100644
--- a/src/gb/gprs_ns2_fr.c
+++ b/src/gb/gprs_ns2_fr.c
@@ -161,10 +161,10 @@
 	struct priv_bind *priv;
 	struct msgb *msg, *msg2;
 
-	OSMO_ASSERT(gprs_ns2_is_fr_bind(bind));
 	if (!bind)
 		return;
 
+	OSMO_ASSERT(gprs_ns2_is_fr_bind(bind));
 	priv = bind->priv;
 
 	OSMO_ASSERT(llist_empty(&bind->nsvc));