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
diff --git a/src/hnbgw_cn.c b/src/hnbgw_cn.c
index 0e993e2..4e5b2e0 100644
--- a/src/hnbgw_cn.c
+++ b/src/hnbgw_cn.c
@@ -409,4 +409,5 @@
 	osmo_sua_user_destroy(cnlink->sua_user);
 out_free:
 	talloc_free(cnlink);
+	return NULL;
 }