rua: Fix null ptr access in hnb->llsk.srv

At startup, osmo-hnodeb connects the Iuh socket to the HNBGW, and start
the LLSK unix socket link_server waiting for lower layers to connect.
If for some reason the lower layers don't connect and an Iuh messages
arrives which was to be forwarded to LLSK, it would try to access it
without first checking if it was already available.
This patch adds a check to avoid a crash when accessing the null
pointer.

Change-Id: I32d8f91b2fc5401ace386e1085c248476228cd18
---
M src/osmo-hnodeb/rua.c
1 file changed, 22 insertions(+), 0 deletions(-)

diff --git a/src/osmo-hnodeb/rua.c b/src/osmo-hnodeb/rua.c
index e109ae0..d705c7c 100644
--- a/src/osmo-hnodeb/rua.c
+++ b/src/osmo-hnodeb/rua.c
@@ -115,6 +115,11 @@
 	ranap_buf_len = ies.ranaP_Message.size;
 
 	LOGP(DRUA, LOGL_DEBUG, "Rx RUA UDT ranap_len=%zu\n", ranap_buf_len);
+	if (!hnb->llsk.srv) {
+		LOGP(DRUA, LOGL_NOTICE, "Discarding rx RUA UDT ranap_len=%zu due to lower layers not available\n",
+		     ranap_buf_len);
+		goto free_ret;
+	}
 
 	LOGP(DLLSK, LOGL_DEBUG, "Tx IUH-UNITDATA.ind ranap_len=%zu\n", ranap_buf_len);
 	iuh_prim = hnb_iuh_makeprim_unitdata_ind(ranap_buf, ranap_buf_len);

