hnbgw: Fix crash if cnlink fails to connect

Return NULL in the error code path, otherwise an uninitialized pointer
is returned and later accessed when a UE tries to register using the
cnlink.

Change-Id: I4f3e2e0680de3216e2e569958bd64f70dc30c2a3

src/hnbgw_cn.c

@@ -409,4 +409,5 @@ out_user:
 	osmo_sua_user_destroy(cnlink->sua_user);
 out_free:
 	talloc_free(cnlink);
+	return NULL;
 }